McMahon died shortly after midnight at Ronald Reagan UCLA Medical Center surrounded by his wife, Pam, and other family members, said his publicist, Howard Bragman.
Bragman didn't give a cause of death, saying only that McMahon had a "multitude of health problems the last few months."
McMahon broke his neck in a fall in March 2007, and battled a series of financial problems as his injuries prevented him from working.
Doc Severinson, "Tonight" bandleader during the Carson era, said McMahon was a man "full of life and joy and celebration."
"He will be sorely missed. He was one of the greats in show business, but most of all he was a gentleman. I miss my friend," Severinson said in a statement.
David Letterman paid tribute to McMahon as a "true broadcaster" and key part of Carson's show.
"Ed McMahon's voice at 11:30 was a signal that something great was about to happen. Ed's introduction of Johnny was a classic broadcasting ritual reassuring and exciting," Letterman said, adding, "We will miss him."
Letterman's bandleader, Paul Shaffer, said McMahon "defined professionalism in broadcasting."
On CBS News' "The Early Show " Tuesday, Mary Tyler Moore discussed knowing McMahon.
"He will be missed," Moore said. "...I was such an admirer, am still, of course, an admirer of Ed McMahon."
Moore, who appeared on "The Tonight Show" herself, said McMahon, who played straight man to Carson's antics, could always hold his own.
Moore said, "Boy, did he not come up with a bunch of good laughs himself."
Jerry Digney, who was McMahon's longtime publicist, said McMahon was the most "courtly, good-natured person you could ever meet" and that he brought "elegance, humor and a new sense of importance" to the role of second banana.
McMahon and Carson had worked together for nearly five years on the game show "Who Do You Trust?" when Carson took over NBC's late-night show from Jack Paar in October 1962. McMahon played second banana on "Tonight" until Carson retired in 1992.
"You can't imagine hooking up with a guy like Carson," McMahon said in an interview with The Associated Press in 1993. "There's the old phrase, hook your wagon to a star. I hitched my wagon to a great star."
McMahon, who never failed to laugh at his Carson's quips, kept his supporting role in perspective.
"It's like a pitcher who has a favorite catcher," he said. "The pitcher gets a little help from the catcher, but the pitcher's got to throw the ball. Well, Johnny Carson had to throw the ball, but I could give him a little help."
"And now h-e-e-e-e-e-ere's Johnny!" was McMahon's trademark opener for each "Tonight" show, followed by a small, respectful bow toward the star. McMahon's style was honed during his youthful days as a carnival hawker.
The highlight for McMahon came just after the monologue, when he and Carson would chat before the guests took the stage.
"We would just have a free-for-all," he said in the AP interview. "Now to sit there, with one of the brightest, most well-read men I've ever met, the funniest, and just to hold your own in that conversation. ... I loved that."
When Carson died in 2005, McMahon said he was "like a brother to me," and recalled bantering with him on the phone a few months earlier.
"We could have gone on (television) that night and done a 'Carnac' skit. We were that crisp and hot."
McMahon's medical and financial problems kept him in the headlines in his last years. It was reported in June 2008 that he was facing possible foreclosure on his Beverly Hills home. By year's end, a deal was worked out allowing him to stay in his home, but legal action involving other alleged debts continued.
Among those who had stepped up with offers of help was Donald Trump.
"When I was at the Wharton School of Business I'd watch him every night," Trump told the Los Angeles Times in August. "How could this happen?"
McMahon even spoofed his own problems with a spot that aired during the 2009 Super Bowl promoting a cash-for-gold business. Pairing up with rap artist MC Hammer, he explained how easy it is to turn gold items into cash, jokingly saying "Goodbye, old friend" to a gold toilet and rolling out a convincing "H-e-e-e-e-e-ere's money!"
Born Edward Leo Peter McMahon Jr. on March 6, 1923, in Detroit, McMahon grew up in Lowell, Mass. He got his start on television playing a circus clown on the 1950-51 variety series "Big Top." But the World War II Marine veteran interrupted his career to serve as a fighter pilot in Korea.
He joined "Who Do You Trust?" in 1958, its second year, the start of his long association with Carson. It was a partnership that outlasted their multiple marriages, which provided regular on-air fodder for jokes.
While Carson built his career around "Tonight" and withdrew from the limelight after his retirement, McMahon took a different path. He was host of several shows over the years, including "The Kraft Music Hall" (1968) and the amateur talent contest "Star Search," whose competitors included over the years Justin Timberlake, Usher, LeAnn Rimes, Adam Sandler and Rosie O'Donnell.
He was a longtime co-host of the Jerry Lewis Muscular Dystrophy Association Telethon, a Labor Day weekend institution, and was co-host with Dick Clark of "TV's Bloopers and Practical Jokes."
McMahon and Clark also teamed up as pitchmen for American Family Publishers' sweepstakes, with their faces a familiar sight on contest entry forms and in TV commercials. McMahon was known for his ongoing commercials for Budweiser as well.
He had supporting roles in several movies, including "Fun With Dick and Jane" (1977) and "Just Write" (1997). He took on his first regular TV series job in the 1997 WB sitcom "The Tom Show" with Tom Arnold.
McMahon released his autobiography, "For Laughing Out Loud: My Life and Good Times," in 1998. In it, he recounts the birth of "Tonight."
"Let's just go down there and entertain the hell out of them," Carson told him before the first show. Wrote McMahon: "That was the only advice I ever got from him."
In 1993, he recalled his first meeting with Carson after they left "Tonight."
"The first thing he said was, 'I really miss you. You know, it was fun, wasn't it?"' McMahon recalled. "I said, 'It was great.' And it was. It was just great."
Besides his wife, Pam, McMahon is survived by children Claudia, Katherine, Linda, Jeffrey and Lex.
Bragman said no funeral arrangements have been made.