They all kinda left sporatically...
Main cast departures
Sherry Stringfield's first departure on the show was in 1996, when her character, Susan Lewis, moved to Phoenix, Arizona, in the Season 3 episode "Union Station." In 2001, Stringfield returned to the series, reprising her role of Dr. Lewis, in the Season 8 episode "Never Say Never." She departed again in the Season 12 premiere, "Canon City." This second departure was not depicted, but rather mentioned by character Kerry Weaver four episodes later in "Wake Up," when she explains that Susan had accepted a tenure-track position at a hospital in Iowa City, Iowa, after having been denied tenure at County.
After playing Anna Del Amico for one season, Maria Bello was simply not a part of the ensemble cast when Season 5 began. Like Susan Lewis's second exit, her character's departure was mentioned but not depicted; in the season premiere, "Day for Knight," Carter explains to new medical student Lucy Knight that the locker she is inheriting used to belong to Anna Del Amico, who is working in a pediatric ER back in Philadelphia, where she has family and a boyfriend.
George Clooney left the show in 1999, in the Season 5 episode "The Storm, Part 2," when his character, Doug Ross, quit before being fired by County for his involvement in a patient's death. Clooney made a brief reappearance in the Season 6 episode "Such Sweet Sorrow" when his character reunited with Carol Hathaway.
Gloria Reuben departed early in Season 6, in the episode "The Peace of Wild Things," when her character, Jeanie Boulet, decided to become a stay-at-home mom and care for her newly adopted HIV-positive baby. She returned in season 14 for one episode with her HIV positive child.
Kellie Martin, who played medical student Lucy Knight, left the series midway through Season 6 in the episode "All In The Family," when her character was killed by a patient suffering from undiagnosed schizophrenia; his psychotic break occurred before a backed-up psychiatry department could arrive in the ER for a consult.
Julianna Margulies left the show at the end of Season 6, in the episode "Such Sweet Sorrow," when her character, Carol Hathaway, decided on the spur of the moment to go to Seattle, Washington, and reunite with Doug Ross, her true love and the father of her twin daughters.
Erik Palladino departed early in Season 8, in the episode ("Never Say Never") after his character, Dave Malucci, was fired for inappropriate conduct.
Eriq La Salle's character, Peter Benton, departed in the Season 8 episode "I'll Be Home For Christmas". He took a job with a set schedule at another hospital in order to spend more time with his son, Reese, and his girlfriend, former fellow ER doctor Cleo Finch. His character returned two more times that season in episodes "It's All In Your Head" and "On The Beach".
Michael Michele's character, Cleo Finch, departed in the Season 8 episode "I'll Be Home For Christmas" with her boyfriend, and fellow doctor, Peter Benton, after having previously taken a job at the same hospital with set schedules (in fact it was she who arranged the interview which resulted in Benton's job offer). Her character returned one more time that season in the episode "On The Beach".
Anthony Edwards's character, Mark Greene, died of a brain tumor in Season 8's penultimate episode, "On the Beach." Unusually, Anthony Edwards was credited in the following episode "Lockdown".
Paul McCrane's character, Robert Romano, whose arm had been severed just above the elbow by a helicopter's tail rotor in the Season 9 premiere "Chaos Theory," was killed in the Season 10 episode "Freefall" when a helicopter that was taking off from the hospital roof was buffeted by strong winds, causing it to crash on the roof and plummet over the side of the building; it fell into a crowded ambulance bay and landed squarely on Romano.
Sharif Atkins left the series twice. The first time was in the Season 10 episode "Where There's Smoke," when his character, Michael Gallant, revealed that the Army was sending him to Iraq. He returned for two episodes in Season 11 and for four episodes in Season 12, where in the episode "The Gallant Hero and The Tragic Victor," he was killed by an improvised explosive device while serving a second tour in Iraq.
Alex Kingston's character, Elizabeth Corday, left the series in the Season 11 episode "Fear" after getting in trouble for performing an illegal organ donation procedure; rather than being summarily fired, County offered her a demotion to a non-tenured position, but she turned it down and opted to return to England instead. In an interview with Britain's Radio Times magazine, Kingston spoke of being written off the show due to her age, a statement that sparked some controversy. She later withdrew that claim.[5]
Ming-Na left in Season 11 as an attending physician when her character, Jing-Mei Chen, resigned in in order to take care of her ailing father, whom she later euthanized in the episode "Twas the Night." This was the second time her character left County General; in Season 1, medical student "Deb" Chen recurred in an eight episode story arc which concluded with her leaving medical school after deciding she was better suited to research than to applied medicine.
Noah Wyle left in the Season 11 finale, "The Show Must Go On." His character, John Carter, after having received tenure at County, decided to reunite with his girlfriend Kem Likasu (portrayed by Thandie Newton). During Season 12, Wyle returned as Carter and made guest appearances in four episodes: "Quintessence of Dust," "Darfur," "No Place to Hide," and "There Are No Angels Here." He is reported to make further guest appearances in season 15.
Laura Innes left midway through Season 13, in the episode "A House Divided," when a reluctant Luka Kovač was forced by budget cuts to fire her character, Kerry Weaver. After bracing for a battle to keep her position, and in spite of the fact that Kovač realized he must find a way to keep her on staff, Weaver ultimately decided to resign from County and accept a job offer from a television station in Miami, Florida.
Shane West left at the end of the 13th season after his character, Ray Barnett lost both his legs and returned to Baton Rouge with his mother to recuperate. West's final appearance as a main cast member was in "The Honeymoon is Over." He will return for one episode in Season 15.
Goran Visnjic is expected to leave the main cast early in the 15th season. As the 14th season opened, his character, Luka Kovač was in his native Croatia to be with his father, who required surgery for cancer followed by an extended recuperation. His character remained active through dialogue and discussion of the status of his stay in Croatia, including a brief appearance in episode 5. When Kovac returned to the US, Visnjic appeared in episodes 8-10 before his character returned to Croatia for his now-deceased father's funeral. He returned again in episodes 14, 16 and 19 to complete his medical storyline, and is expected to make his final appearances early in Season 15 to wrap up Kovac's story. [3]
Maura Tierney will depart early in the 15th season, along with her TV husband Goran Visnjic