NYPD Detective Pete McMahon has seen it all.

In his 38-year career — coming to a close with his retirement on Thursday — the venerable homicide detective has investigated hundreds of murder across four boroughs and had a front-row seat to the city’s resurrection.

“I can’t believe how much the city has changed in 38 years. … I laugh now when detectives talk about how busy we are,” said McMahon. “There were nights in the past when I was working and I would catch two murders in the same shift.”

McMahon, 62, has been patrolling the streets of New York for so long that he’s the last active cop who didn’t attend the police academy, but rather the Housing Authority academy, back when the NYPD, NYCHA and MTA had separate forces with separate academies.

He came on the job in April 1981 and was assigned to the public housing complexes of the drugged-out wasteland that was then the Lower East Side.

“The streets were full of heroin addicts walking around like zombies,” recalled McMahon. “The streets were full of addicts and drug dealers.”

McMahon made detective in 1986 and the next year cracked his first homicide, a slaying in Harlem’s Taft Houses.

“I was so happy when I got that confession,” remembered McMahon. “I must have had a big smile on my face and the suspect said to me, ‘Wow, you are happy.’”

McMahon was hooked — and he was about to get more work than he could ever want.

In 1989, he was assigned to investigate homicides in housing projects across Manhattan and the Bronx, just in time for some of the bloodiest days of the crack epidemic.

“In the early ’90s, we had over 2,000 murders a year,” said McMahon. “Now we have under 300.”

By 1992, McMahon was shifted to working murders in Queens and Brooklyn, landing in the latter borough’s homicide squad for good when housing cops were moved under the umbrella of the NYPD in 1995.

In a way, it was a homecoming for McMahon, who grew up in south Brooklyn inspired by his cop uncle, and lives there to this day.

“It is an honor to be part of a police department that has made the city where I grew up and raised my family that much safer,” said McMahon.

Over the years, McMahon was personally assigned more than 200 murders and assisted on over 1,000 more. He has worked homicides longer than any other cop currently in the department.

He was there in 1993 when Stephan Poole was nabbed for strangling his 4-year-old foster son to death for crying, then dumped the tot’s body into the trash compactor of their East New York building.

He was there in the early minutes of Jan. 1, 2000, when Dickens Desir was gunned down on Ocean Avenue, becoming the city’s first homicide victim of the 21st century.

He was there for eight police murders, the worst days of a career that saw McMahon stare death in the face daily.

“Those were definitely low points, but I was able to bring some solace to the families,” said McMahon.

The fatal shooting of off-duty auxiliary cop Francky Aleger as he walked from his Canarsie home to the train in 2012 remains unsolved, marking it as one of McMahon’s biggest regrets, he said.

As he entered the homestretch of his career, one of the cases that kept McMahon coming back year after year was the fatal 2009 bludgeoning of Brighton Beach businessman Vladimir Tolstykh.

Tolstykh’s alleged killer, Abakar Gadiyev, was on the lam for years, living openly in Australia, as McMahon battled through extradition bureaucracy to bring him to justice for Tolstykh’s widow.

“Every summer I kept saying this is it,” said McMahon. “But then I thought, ‘How could I tell Rita?’”

His doggedness finally paid off in February 2019 when Gadiyev was finally flown back to New York for his arraignment.

McMahon’s nearly four-decade career has left him with a story to tell at countless red lights he and his patient wife find themselves sitting at.

“When I start to talk about a shooting on that corner, my wife just rolls her eyes,” he said.

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McMahon said that one of the biggest factors he’s seen in the city’s turnaround is the advent of modern technology, including the proliferation of video surveillance and DNA testing — but there’s no substitute for good, old-fashioned police work.

“A detective recently told me he did a video canvass. I asked, ‘Did you talk with any witnesses?’” said McMahon. “He said no. Videos are great, but witnesses are important.

“When I was a young detective, I learned a lot from the older, more experienced detectives,” he said. “I hope during my career I was able to help younger detectives. You can’t teach experience.”

McMahon said that’s what he’ll miss the most.

“Life is easy when you are doing something that you like,” he said. “You enjoy going to work, and that has been my career.”

Additional reporting by Aaron Feis