Carter's career path is one of the main story arcs of the series, beginning as a third-year medical student, becoming a resident, first in surgery and then in emergency medicine, before being promoted to an attending physician.
His career got off to a rocky start when on his first day at County, he nearly vomited in the emergency room after seeing a critically wounded patient and had to be consoled by Chief Resident Dr. Mark Greene (Anthony Edwards).
He was part of an influential and wealthy family and did not need a salary, enabling County General to take him on despite the lack of funding for an additional position.
As a result of Carter's chronic battle with pain, survivor guilt, and resistance to getting help, he eventually develops an addiction to narcotics.
After Abby Lockhart (Maura Tierney) catches him injecting left-over fentanyl from a trauma into his wrist, Carter's colleagues hold an intervention and Dr. Greene demands that he go to an inpatient rehab center for medical doctors in Atlanta or be fired.
Upon returning from rehab in season 7, Carter makes peace with Chase, and apologizes for his long absence, saying, "I didn't want to admit to the fact that I was just like you."
At the end of the season, Kerry Weaver (Laura Innes) rejects Carter's application for Chief Resident because of his history of addiction, stating that his spiral is too recent for him to take on the pressure of that hugely difficult position.
However, early in Season 8 a combination of some reflection by Kerry and a gruesome error by Chen on a case that ended with a patient's death leads her to change her mind and offer Carter the CR spot, which he accepts.
These personal issues come to a head when Abby's bipolar brother Eric (Tom Everett Scott) reappears the day Gamma dies.
Unable to shake his grief, Carter decides to join Dr. Luka Kovač (Goran Visnjic) in Congo without Abby's support.
Carter remains in Africa for several months and works in Makemba "Kem" Likasu's (Thandiwe Newton) AIDS clinic.
Carter and Kem develop a relationship, and when she gets pregnant with his child he asks her to accompany him to Chicago, where he introduces her to his colleagues.
During season 11, Carter starts building an HIV/AIDS clinic adjacent to County General, with funding from his family's charity foundation.
In season 12, Carter appears in a four-episode arc, working with a fellow doctor in Darfur, Sudan, where he is joined by Dr. Greg Pratt (Mekhi Phifer) and Debbie (Mary McCormack).
Carter indicates he might come back to County for good; however, this is potentially contradicted by his earlier idea that he would leave Chicago if it would save his marriage.
Throughout the course of the series (particularly at the beginning of season 8) we meet various members of Carter's wealthy family and see that he is very uncomfortable with coming from money, to the point where he goes out of his way to not talk about his background.
His father, John (Jack) Truman Carter Jr. (Michael Gross) is caring but reserved, and very acquiescent with his wife until he gets tired of her being an "emotional vampire" and divorces her.
Carter eventually figures out that much of her sadness comes from this and they briefly become closer, but eventually Eleanor cut ties with her family after Jack divorces her, to the point where John has no idea how to reach her and she refuses to return a phone call he makes after his grandmother (who never approved of Eleanor) passes away.
They do occasionally argue, however, usually in regard to Carter's reluctance to participate in matters related to the family foundation; Gamma modifies her will to remove Jack as the head of the foundation and puts John in charge of it, leading to him changing its mission from supporting Chicago arts programs to supporting public health initiatives.