Peter Mills is currently living in North Carolina with his mother and sister, running their own restaurant. Introduced in Season 1 as the new Fire Candidate on Truck 81, Mills has proved himself to be a determined and skilled firefighter. He was eventually transferred to Rescue Squad 3, however due to illness was transferred to Ambulance 61 as he was not cleared for firefighter duty. He was eventually reinstated as a firefighter and transferred back to Squad 3, where he later decided he did not want to be a firefighter and wanted to be with his family in North Carolina.
Mills' father was also a firefighter, but was killed after he and Boden rushed into a burning building against orders. His mother and sister owned and operated a diner, where Mills occasionally worked while off-duty. The diner was eventually destroyed by a fire set by Kevin Hadley. At the end of Season 1, Mills applies to the Chicago Police Department academy, suggesting that he plans to leave Firehouse 51 after finding out Boden and his mother once had an affair.
Mills and Gabriela Dawson share a love of cooking, which brings them together in the kitchen often. On one occasion, they share a home-cooked meal as 'friends' which ends up being much more. The two start a secret relationship, then later on decide to come out as a couple. However, Mills breaks off their relationship after Gabriela reveals to him that she knew about his mother and Chief Boden, Mills feeling as though he cannot trust her. Mills attempts to reconcile with Gabriela if she can assure him that she has no feelings for Matthew Casey, when she admits that she can't their relationship seems over for good. Even though Mills and Dawson's relationship is over, they still maintain a good working relationship and friendship.
Mills and Sylvie Brett had a very good friendship, in which they both often relied on one another since they worked as partners, Sylvie was upset when she found out Mills was moving to North Carolina near the end of Season 3 and during his goodbye party, she breaks down in tears and hugs Mills goodbye
Mills was assigned to Truck 81 under Lieutenant Casey at firehouse 51, where his father Henry Mills had worked until his death in a fire during a call with Chief Boden . From then on, Mills was determined to become a firefighter, something his mother had a hard time accepting.
During his time under Casey, he develops a good relationship with Lieutenant Severide and not so much with Casey. Severide eventually wants to transfer Peter to Squad 3, which doesn't go down well with Casey and the rest of 81 as they feel he isn't ready. Peter also starts to date Gabriela Dawson.
Severide's father Benny also comes to visit 51 after he's revealed to be a former firefighter who served there with Henry Mills and Chief Boden. There's some tension between Boden and Benny about how Henry died when Benny rejects the idea of Henry dying as a hero. Boden tells him to leave and not come back. During a dinner dedicated to firefighters, Boden praises Peter for recent work, where Benny gets up and leaves midway. Afterwards, the 2 meet up and Benny tells Boden he knew about his affair with Ingrid (Henry's ex-wife at the time) which was the reason Henry rushed in and died. Dawson hears this. They fight and Boden later tells Severide to tell Benny not to come back to 51. He does so anyway where he meets Peter who asks him persistently why he doesn't accept the story about Henry. Benny shouts at him and says Henry wasn't a hero, that he took off his oxygen mask midway the call, causing another firefighter to go after him, killing them both. This angers Mills and the others hold him back.
Peter and Dawson's relationship strengthens but Dawson holds back as she still isn't sure whether she should Peter about the conversation about the affair. She tells him eventually and Peter breaks up with her for not telling him earlier. She's upset and heartbroken but In 1 x 23, he tells her he's willing to move on and get back if she tells him she doesn't have feelings for Casey. She's unable to and they break up.
Peter applies to the Police Academy at the end of season 1.
Season 2 Edit
Antonio tells Peter he saw his name on the list and he'll soon be a policeman. However, he rejects this later on, during a mission where he ends up being trapped with Boden. He tells him that he was upset with Boden but that it was a waste of time and he shouldn't have.
Mills is also finally transferred and accepted in Squad 3 after he finishes his training and courses.
He also has a brief relationship with Rebecca Jones while she is a candidate in Squad 3, she connects with him the most and he often gives her a lot of advice on how to cope, being the recent candidate as most the others refuse to accept her because of her cocky behaviour. When she commits suicide, he is deeply disturbed by it and snaps when Hermann and the others talk about her coldly. He tells them they could've been nicer since they were always judgemental of her. Hermann doesn't take this well and the 2 fight for a while until Hermann admits that he saw her the night before and he wonders whether he should've stepped up and talked to her.
The season ends with Squad 3, Truck 81 and Dawson and Shay inside a broken building trying to clear the building out while Boden directs them outside. The building suddenly explodes.
Season 3 Edit
Beginning with the fire at the end of season 2, they find everyone but Mills, who they later find with a broken leg. Some time later, he's cleared for duty and returns to 51.
Mills goes on a mission to try and find and talk to his father's side of the family as he never knew them. He gets the new transfer, Rick Newhouse, to track them down. When he finds them, they welcome him coldly. His mother tells him that they didn't like her marrying Henry as she was mixed race and he was white and tried everything to get rid of her. The person he met finds him at 51 and tells him that his granddad has agreed to see him but he tells him to get lost.
Peter experiences blackout, and after falling off the top of the truck, Severide gets worried and tells him to get checked out. The doctor tells him he can't clear him for duty and that he can't be a firefighter anymore. He's upset but quickly gets past it and takes Dawson's place as a paramedic when she moves to Truck 81 as the new candidate, alongside the new transfer who took Shay's place, Sylvie. He helps her when she experiences her first time when she isn't able to save someone, the two become friends.
Later on, Boden tells Peter that during an arson investigation, they found that the fire killing Henry and the one on Shay were connected. They catch the arsonist in a crossover between PD and Fire and he dies.
Peter stays a paramedic until he finds out that he has been cleared for Squad 3 in 3x18 following a medical exam. His sister tells him to leave the job to work at a restaurant that Leonardo, his grandfather, left them in his will. She tells him the job damaged his own health and killed their father. However, he is cleared by his medical and joins Squad 3 again. His mother and sister tell him they want to go to North Carolina to start up the restaurant with another shareholder.
Eventually, Peter takes time to make the decision that he wants to leave to North Carolina with his family. He tells Chief that his father would have been proud he was a firefighter and followed in his footsteps but he would want him to live his life too. He's adamant on his decision, which surprises everyone at 51 since he just got back on squad. Chief expands Squad for one shift for Mills and 51 accept it and throw him a going away party at Molly's. Everyone is upset by the decision though; Chief tells him to rethink, Hermann gets angry, Dawson tells him she doesn't understand his logic, Cruz also questions him on his decision. This leaves Jessica Chilton, the new replacement paramedic, to receive a cold welcome - Otis tells Chief they can't just get over Mills leaving after serving 3 years. Chief snaps and tells them to get over it.
After he leaves, Peter sends the house a box of treats and a photo of him and his family standing out their newly opened restaurant. Everyone still misses him a lot.
(Mills to Dawson) - "Am I getting in the way of you and Casey?"
(Dawson) - "Absolutely not, he's just a friend"
Mills to Chief Boden: “Chief. Respectfully, shut up.”
Mills to Chief Boden: "I don't know if my dad expected me to follow in his footsteps, I do know that he would be proud I did."
Mills to Chief Boden: "I can't keep chasing his (Henry) ghost".
|Season 1 Appearances|
|"Pilot"||"Mon Amour"||"Professional Courtesy"||"One Minute"||"Hanging On"|
|"Rear View Mirror"||"Two Families"||"Leaving the Station"||"It Ain't Easy"||"Merry Christmas, Etc."|
|"God Has Spoken"||"Under the Knife"||"Warm and Dead"||"A Little Taste"||"Nazdarovya!"|
|"Viral"||"Better to Lie"||"Fireworks"||"A Coffin That Small"||"Ambition"|
|"Retaliation Hit"||"Leaders Lead"||"Let Her Go"||"A Hell of a Ride"|
|Season 3 Appearances|
|"Always"||"Wow Me"||"Just Drive the Truck"||"Apologies Are Dangerous"||"The Nuclear Option"|
|"Madmen And Fools"||"Nobody Touches Anything"||"Chopper"||"Arrest in Transit"||"Santa Bites"|
|"Let Him Die"||"Ambush Predator"||"Three Bells"||"Call It Paradise"||"Headlong Toward Disaster"|
|"Red Rag the Bull"||"Forgive You Anything"||"Forgiving, Relentless, Unconditional"||"I Am the Apocalypse"||"You Know Where to Find Me"|
|"We Called Her Jellybean"||"Category 5"|