At the beginning of the season, Casey's friend, Andrew Darden, dies in a flash fire, with Casey being very upset about his death. He gets blamed by Severide for it, and he in turn blames Severide, saying Darden never should have been allowed to go into the building. Eventually, the two work out their differences and go back to being friends and working together as the two Lieutenants in the Firehouse.
Casey is in a relationship with Hallie Thomas and is engaged to her but the two later break up. However, Casey later gets back together with Hallie when she returns from a long trip in South America.
He had a rough relationship with Detective Hank Voight, after attending a call where his son, Justin Voight, was a drunk driver involved in a car accident which left a young boy paralysed. Casey, even after being bribed by Voight to lie about what he saw, ended up testifying to the truth, causing problems in the firehouse.
After Hallie and Casey get back together, they happy until she unexpectedly dies in a fire at the clinic she worked at part time, in the season finale. It was his firefighters at the scene, and him who managed to take her out. However, despite all their efforts, Hallie dies at the hospital later on. He's very devastated by her death, and is comforted by Kelly Severide and tries to dig deeper and find out what happened with the help of Antonio.
After Dawson breaks up with Mills, she helps Casey through the death of Hallie and the 2 start dating later on in the season.
Heather, meanwhile, ends up in a car crash, killing her best friend who was with her at the time. She asks him to take care of her children while she's away for a year and a half in prison. He agrees for Andy. Gabriela helps him through it until Heather is released. She thanks him and the kids have grown close to him but they decide to move away.
At the end of season 2, Casey proposes to Dawson but they get a sudden call for a fire. They leave the proposal unanswered. The building explodes.
Begins with Shay, who dies at the scene from a head injury, leaving Dawson and Severide distraught. Dawson comes back after a month's leave. She tells Casey she couldn't accept his proposal because it would be the day that Shay died and she can't mix the memory with such a sad one. They get engaged later on when Casey tells her that Shay picked out the ring with him.
Dawson's eventually transferred to work under Casey in Truck 81 and so they can't get married as she can't work under him as long as she's a candidate. It works out great until Dawson makes a mistake, causing Casey to call her out and threaten to kick her off truck since she ignored his instructions. She's fine with it until Casey refuses to talk to her after shift as he's still upset with her. She works late at Molly's that night and asks Peter to help her. The two start talking and she tells him she's finding it hard to talk to him after shifts since the job is mixing in. Casey comes by to talk to her but finds the 2 and takes it the wrong way and leaves. Peter tells him he doesn't want to get in the way the next morning and Gabriela apologises too but he ignores her. She decides to take a break even though he doesn't want to and leaves the house, moving in with Sylvie for the meantime.
After Chief Boden's father passes away, he decides to take a break from the job for a few months, leaving Chief Pridgen in charge. Even though he seems friendly at first, he constantly mocks the team, specifically Otis. This angers Casey and he speaks up against him. Combined with this, Casey has a brief relationship with Pridget's newly divorced ex-wife. He ends it when he finds out but Pridgen finds out, taking out his anger on Casey on the job. During a fire, Truck 81 and Squad 3 are about to go in but Pridget deliberately redirects Truck 81 to a minor accident nearby (a man who fell down the stairs). This angers Casey but Pridgen refuses to let him work at the fire. The team leaves while Squad 3 takes care of the fire to rescue the trapped girl inside. During this, another man has rushed into help and is on the top floor but they miss him. Casey comes back and tells Pridgen that the man refused treatment and he was fine. Just then, a man falls out the top floor out of the window. The man then takes action against firehouse 51 for negligence. The case is investigated and Welch and Severide hear Pridgen tell them that Casey failed to check and that the reason he was redirected was because there was a 'life threatening' injury. Severide files another report, going against the initial statement made by Pridgen. He finds out and encourages Severide to get the promotion to Captain in OFI or be stripped of his rank but Kelly refuses and claims he's fine where he is. Things worsen since the man refused treatment and there's no paperwork indicating that Casey even went to help. Pridgen then uses Welch's vendetta towards 51 to get him to sign a statement to support him. He refuses due to respect for the people there and is kicked out. However, Welch goes on to make a statement backing Severide, resulting in Pridgen getting fired and Boden returning to take control.
Late in the season Casey decides to find some construction work off shift. He finds a club called Stilettos where the owner, Jack Nesbitt, was looking to expand. Casey reveals to Nesbitt that he is a firefighter and Nesbitt does the same. Jack was a former firefighter at Firehouse 114 for seven years before quitting. Casey later asks Scott Rice, who also previously worked in 114, about Nesbitt where Rice revealed Nesbitt had some bad history with some department officials, therefore leading to his resignation. Casey then got jammed up in Nesbitt's business in which he was illegally trafficking European girls into the country. After an undercover job, the girl Casey befriended from the club, Katya, was discovered dead in his apartment by Dawson who calls Antonio to track Casey down. Casey is found with Nesbitt and two other men at the club where a fight broke out and Casey is beating up Nesbitt. Nesbitt is immediately arrested but later released after it is discovered he is working as a CI with the FBI on the case. However, Jack is later arrested again by the FBI and CPD after tailing Casey one night.
At the start of the season, Gabby learns she is pregnant and leaves her job as firefighter to take an office job. However, her and Casey's happiness is short-lived. Gabby and Boden are at the office trying to find files on a case when she cries out in pain. She's taken to hospital quickly and within a few hours, Doctor Halstead [Chicago Med] reveals that she's miscarried. She's broken up about this and he tells her that the baby never had a chance. Him and Gabby have this rough patch in their relationship.
During a tornado in Chicago, powerlines are cut and houses destroyed. The 52nd state Alderman of Chicago encourages Casey, who was at the scene and helped take many of the victims to the only homeless shelter in the area, to make a speech to drive in donations. He is happy to do so and helps to raise thousands of dollars. However, when he returns to the shelter, many of the people are still there and have not recieved any help or donations. Casey talks to the Alderman, who brushes him off, implying he has taken the money for his own personal use. He tells Casey that it's just 'politics'. Angered by this, he decides to run for Alderman himself, after a lot of encouragement from Gabby.
However, during his campaigning, he finds he needs to do more than make promises. He is constantly overthrown in his decisions and shown under badlight by others. He is frustated even more so when someone covers cars and billboards claiming that Casey is a 'murderer's son'. Knowing his oponent did this, he threatens him to take the posters down but he is dismissed, causing Casey to think about withdrawing himself from the elections. However, Gabby tells him to keep going and is upset to hear that he is thinking of quitting, which encourages him to continue.
Casey finally gives in and talks to a gang-leader who had promised to help him win if they took down cameras from the street-corners to let them deal drugs. Initially, he was against this idea but agrees to do so, asking them just for one thing - not to hang around the corners around the children's high school. They aren't sure about this but imply that they are willing to do so. However, this still isn't enough, as he loses his first debate to his oponent, and then is informed about a second debate that no one told him about. However, as his campaign affects his ability to work, he is told by Chiefs to stop or not miss any time off work. He therefore agrees and deliberately misses the second debate as he is on a call. His oponent calls him weak and says that Casey is not willing to give up enough for his job as Alderman as he still has a second job. This stresses him out but the media catch him risking his life for a mechanic and publish it in the newspapers as the reason for him missing the debate. This heroism helps him to win the four-year term as Alderman.
Later on, Truck 81 respond to a call of a person in distress. Upon arrival, they find that the victim is a high-school boy. They help him down and he fourtunately survives. The boy's grandmother then shows up at the firehouse and tells Casey to do more as Alderman. He finds from Antonio that the boy was attacked by a gang who wanted revenge on Victor, the boy, because his older brother was previously affliated with the same gang but helped take some of them down when he agreed to help the police. Casey goes to see the gang leader again and tells him to stop terrorising Victor but they are unwilling to. 3 days later, Casey goes to see Victor, who wants to go to graduation. He tells him not to as he could be attacked again, but Victor remains unafraid. Seeing no other option, he gets together with the firehouse and they line up along the road ready to protect him. Chief Boden and Casey personally walk with Victor until he is at the school.
Casey also goes through a difficult time in his and Gabby's relationship after a call causes her to make an unexpected decision. They find a mother with an overdoes and a small 1-2 year old boy in the flat with a fire in the apartment. They help the mother out and Gabby goes in to find the boy. She finds him hiding under the bed and after some time, agrees to come out. When she comes out, the boy is unwilling to let go of her, so she goes with him to the hospital. However, social workers tell her that he was fostered and will be back in a foster home. She feels terrible about him and tells Casey she wants to foster him. However, she had previously told him that she didn't want to get married and so he tells he that it's not such a good idea. She is upset and leaves, but tells him the next morning that she has decided she wants to take the boy in herself and he doesn't have to help her. In the season 4 finale Dawson does get to foster Louie the boy from the flat fire and Casey goes on a trip with his political adviser. Towards the end of the episode she kisses him and writes her room number on a napkin. He then goes to Dawson's new loft attempt that Herman owns and she hugs him. It does it say weather he tells her or not.
The first half of the season mostly deals with Louie and Casey's political career. Gale McLeod - a political advisor - tries to get him to hire her to work on his campaign but he refuses. Just days later, another opposing politician, who had threatened to destabilize Casey's career unless he listened to his own demands, tells the media that Casey used his position as alderman to skip the fostering waiting time for Louie to adopt him. This scares both him and Gabbie as both are threatened with the issue of Louie being taken away again.
Casey decides to call McLeod, who is on the case immediately and solves the issue in no time. However, he soon discovers that McLeod and the politician worked together to trick Casey so that she could use him for her own gains. He fires her afterwards and he goes back home to Louie and Gabby, who he is evidently happy with.
In 04x08, Casey is on a call to solve a young boy trapped in a warehouse after playing and accidentally falling, leaving him hanging upside down for sometime, causing him to stop breathing. Casey and his team rescue him and Casey gives the boy CPR, getting him awake and breathing again. He seems fine until Gabby tells him the boy is in a coma and later, the dad visits him and truly thanks Casey for getting him down. Casey is very upset by this and starts talking about whether he did enough to save the boy, to which Severide responds that he did because he was there and he saw. However, he still continues to think about it and visits the hospital again, where he is tragically told that the boy died. He doesn't tell anyone until Gabby finds out.
Later on, Gabby and Casey are told that they can now adopt Louie if they want, but that Gabby will be seen as a single mother as they are not married. Casey tries to ask around for another option but since they aren't engaged or married, there is none. However, later on at Molly's, they both get the idea of running away and getting married. They do this and the whole of 51 is there to celebrate it. On their way home, Casey sees the man who has been stalking them for the past few days and running away. The man finally comes out and says that he is Louie's father and that he wants him back. In the end, Gabby realises that Louie has a loving family who want him and it would be wrong to deny him of that, so they end up giving Louie to his father for good. This hurts both Gabby and Matt, and Matt wakes up early in the mornings to clear out Louie's room.
Matt was shot in the shoulder in Telling Her Goodbye, after he was held hostage with the rest of 51 when a street gang took refuge there to save one of their own. Sylvie and Gabby were out on a call and came back to find 51 on lockdown. Gabby desperately tried to reach out to Matt after he opened the shutters for a minute and went to call out to her but was shot in the process. It was artificial though and he was not seriously injured.
Casey runs into trouble as alderman, when he tries to push through a bill to increase payouts to firefighters for their medical troubles. His opponent publicly accuses him of trying to "line the pockets" of his fellow firefighters. Casey continually tries to play nice by trying to explain that that's not what he's doing. After being played more than twice, he goes to an alderman meeting during his shift. Outraged, he tells everyone there that first-respondents are those who risk their lives and sometimes don't make it back. He tells them that their payouts don't cover all the medical bills and psychological trauma, especially when a firefighter can't come back. He earns applause and then resigns as alderman, encouraging everyone to vote for the woman who told him to run in the first place.
In My Miracle, Gabriela and Matt have an argument after she invited her father to stay with her because he was having financial troubles. He didn't want him to stay with them and expressed it clearly, which was unlike him. However, Ramón then ended up ruining an important deal with Casey as alderman. He didn't come home that night and told her he wouldn't until Ramon left. Gabriela was left very hurt and tried talking to him, but he told her that she didn't listen to him. However, during that shift, the whole of 51 is called to a warehouse fire.
Gabriela and Sylvie waited outside while five crew members went inside to tackle the blaze, which turned deadly in a matter of seconds. Mouch suffers a heart attack and Herrmann finds himself trapped inside a room without a way out. Soon enough, Jason, Severide and Casey also realise there is no way out. Casey takes off his mask and talks to Gabriela, who tells him to put the mask back on and come out. He can't, and tells her he loves her. She starts to cry, and he tells her "you're my miracle". The episode ends with Gabriela standing in front of the fire while the five are trapped inside.
It is revealed in 6x01 that he survived the warehouse fire from the previous season. He is also awarded a medal for bravery due to his actions at the scene.
He was engaged to Hallie Thomas until her death.
- To Gabriela: "I love you, Gabby. You're my miracle."