How Sam Waterston was written off ‘Law & Order’ after 30 years: ‘It’s been a helluva ride’

Sam Waterston’s Manhattan DA Jack McCoy tried his final case on Thursday night’s “Law & Order” before resigning — but he went out with a flourish and with his storied integrity unbroken.

Waterston, 83, announced earlier this month that he was leaving “Law & Order” after over 400 episodes since joining the granddaddy NBC series in 1994. He’ll be replaced by Tony Goldwyn, 63, who will play DA Nicholas Baxter.

Waterston took center stage on his swan song episode, entitled “Last Dance,” as Jack McCoy tried the headline-grabbing case of a tech billionaire, Scott Kelton (Rob Benedict), accused of killing a woman named Veronica Knight in Central Park who he allegedly assaulted at a company retreat six years earlier.

Kelton was arrested by Detectives Shaw and Reily (Mehcad Brooks, Reid Scott) and taken into custody, where the case fell to ADA Nolan Price (Hugh Dancy). But, since this is “Law & Order,” there’s a wrinkle: turns out that Kelton is good friends with New York Mayor Payne (Bruce Altman), who’s backing McCoy in his re-election bid … as long as he doesn’t involve the mayor’s son, who’s also one of Kelton’s closest friends and confidantes and could have incriminating information implicating him.

Fat chance.

“Politics play absolutely no role in this office,” McCoy proclaims as he orders Nolan to do what he needs to do in order to convict Kelton of the crime (he eventually admits to pushing Veronica and then strangling her). When Mayor Payne invites Nolan to his club for a “friendly” drink, and threatens to destroy both he and McCoy — “My son is off-limits,” he tells Nolan — McCoy pulls Nolan off the case and decides to try it himself. That, in turn, enrages the mayor, who calls McCoy with veiled threats about pulling his support of his reelection campaign.

“I can’t let the mayor’s office dictate how we proceed,” McCoy proclaims. “I told him to go to hell.”

McCoy then steps into the courtroom limelight as the lead prosecutor, laying out his case against Kelton in a flourish of courtroom drama.

The jury comes back with a verdict: Kelton is guilty of second-degree murder, but, for McCoy, it’s a hollow victory.

We’re then taken to a scene with Nolan and McCoy in McCoy’s office post-verdict — when McCoy drops his bombshell.

“I tendered my resignation 20 minutes ago,” he tells an astonished Nolan. “I’ve been thinking about this for a while. It’s time, it just is. The mayor is a vindictive SOB. He’s going to call in every chit he has to make sure my opponent wins the election and he will succeed.

“Then, he’s going to bury everyone who wronged him, including you [Nolan]. If I step aside now, then the governor will be able to appoint someone with integrity.

“It’s been a helluva ride,” McCoy says, toasting Dancy with just the hint of a tear in his eye.

Cut to the episode’s final scene. It’s nighttime now, and McCoy, wearing his hat and coat, briefcase in hand, gazes up at the courthouse with a wistful look in his eye and quietly walks away — his dignity intact as the camera pans upward toward the top of the courthouse.

Fade out.