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.