After accidentally making his stand-up debut in 1977 at El Brookman's in Washington, DC, where he had a pitcher of beer poured on his head, Rooney returned to stand-up comedy, first in New York City and then in Los Angeles, where he became a regular at the Improv comedy club and met Leno, according to a notice submitted by Mandyam.

Rooney co-wrote Leno's first comedy special Jay Leno and the American Dream. “He was the greatest muse a comedian could ever have. I got so much material just through conversations with Kevin. The first time I saw him, I thought, 'Oh boy, this guy is really good.' It's a real voice,” Leno said via Mandyam.

“There’s no gimmick. He didn't have a catch phrase. There wasn't a funny look. When times were rough, they throw out the catch phrase or funny look. Kevin didn't have any of that. I don't know anybody who thought they were better than Kevin Rooney. When Kevin took the stage, we'd all step inside,” Leno added.

Leno further said, “Me, Seinfeld, Larry Miller. At the end of the night, we'd all say, 'Rooney's story was really funny.' We were all genuinely happy when he had success. He was a guy who earned it, a unique voice. Never derivative of anyone else. It was that classic New England sarcastic voice. But there was a humanity behind it. Even when he said the most sarcastic thing. He will be greatly missed."

Rooney made an appearance on both David Letterman's Late Night and The Tonight Show. He contributed articles to Boston Common, My Wife and Kids, Brothers, The Golden Palace, Politically Incorrect, and Til Death. He played a part in Harry Met Sally.

Judd Apatow, who retweeted the Improv's notice of Rooney's passing and wrote that he was "very heartbroken to hear the news," was one of the young comics Rooney coached. "He was hilariously funny. His material was so smart and biting," Apatow wrote in part. "When I first started he was so kind and taught me so much about stand up and writing. A legendary person. RIP."