He was the only astronaut to fly in all three of NASA's original manned spaceflight programs: Mercury, Gemini and Apollo. Although he never walked on the moon, Schirra laid some of the groundwork that made the lunar landings possible and won the space race for the United States.
Schirra died of a heart attack at Scripps Green Hospital in La Jolla, said Ruth Chandler Varonfakis, a family friend and spokeswoman for the San Diego Aerospace Museum.
In 1962, the former Navy test pilot became the fifth American in space -- behind Alan Shepard, Virgil "Gus" Grissom, John Glenn and Scott Carpenter -- and the third American to orbit the Earth, circling the globe six times in a flight that lasted more than nine hours.
Schirra returned to space in 1965 as commander of Gemini 6. Some 185 miles above Earth, he guided his two-man capsule to within a few feet of Gemini 7 in the first rendezvous of two spacecraft in orbit.
On his third and final flight, aboard Apollo 7 in 1968, he helped set the stage for the landing of men on the moon during the summer of 1969.
An inveterate prankster, he could be grumpy and recalcitrant in space, most famously during his Apollo mission.
But "on Mercury, Gemini and Apollo, he flew all three and did not make a mistake," said Christopher Kraft, who was Schirra's Mercury and Gemini flight director and later head of NASA's Johnson Space Center. "He was a consummate test pilot. The job he did on all three was superb."
Of the Mercury Seven, only Glenn and Carpenter are still alive.
Schirra was named one of the Mercury Seven in 1959. Supremely confident, he sailed through rigorous astronaut training with what one reporter called "the ease of preparing for a family picnic."
"He was a practical joker, but he was a fine fellow and a fine aviator," Carpenter recalled Thursday. "He will be sorely missed in our group."
Roger Launius, a space historian at the Smithsonian Institution in Washington, said Schirra "had a personality that was fun and effervescent. He had the gift of gab. He was able to take complex engineering and scientific ideas and translate that to something that was understandable."
Launius recalled that Schirra smuggled a corned beef sandwich onto his Gemini flight. During the mid-December 1965 flight, he and his Gemini crewmate, Thomas Stafford, unnerved Mission Control when they reported, slowly and in deadpan fashion, seeing some kind of UFO consisting of "a command module and eight smaller modules in front. The pilot of the command module is wearing a red suit" -- Santa Claus.
Then Schirra and Stafford played "Jingle Bells" on a tiny, smuggled-aboard harmonica and a set of sleigh bells.
"At times he gave us a hard time during his flight; technically what he did was superb," Kraft said.
Schirra blasted off from Cape Canaveral on Oct. 3, 1962, aboard the Sigma 7 Mercury spacecraft. "I'm having a ball up here drifting," Schirra said from space before making a perfect splashdown in the Pacific Ocean.
His Gemini mission represented a major step forward in the nation's space race with the Soviet Union, proving that two ships could dock in space.
Kraft said Schirra showed great poise during his Gemini flight when a problem on the launch pad cropped up. Schirra would have been warranted in triggering a launch ejection, but instead, he held steady, and the launch went off OK, Kraft said.
Schirra's Apollo mission in October 1968 restored the nation's confidence in the space program, which had been shaken a year earlier when three astronauts, including Grissom, were killed in a fire on the launch pad.
The Apollo 7 crew shot into space atop a Saturn rocket, a version of which would later carry men to the moon. But Schirra and his two fellow crewmembers were grumpy for most of the 11-day trip. All three developed bad colds that proved to be a major nuisance in zero gravity.
The following year, Schirra left NASA and retired from the Navy with the rank of captain, having logged more than 295 hours in space. He became a commentator with CBS.
"Mostly it's lousy out there," Schirra said in 1981 on the occasion of the first space shuttle flight. "It's a hostile environment, and it's trying to kill you. The outside temperature goes from a minus 450 degrees to a plus 300 degrees. You sit in a flying Thermos bottle."
Born in Hackensack, N.J., Schirra was practically born to fly. His father was a fighter pilot during World War I and later barnstormed at county fairs with Schirra's mother, who sometimes stood on the wing of a biplane during flights.
Schirra took his first flight with his father at age 13 and already knew how to fly when he left home for the U.S. Naval Academy in Annapolis, Md.
Schirra flew 90 combat missions during the Korean War. He was credited with shooting down one Soviet MiG and possibly a second. He received the Distinguished Flying Cross and two Air Medals.
In 1984, he moved to the San Diego suburb of Rancho Santa Fe, serving on corporate boards and as an independent consultant. His favorite craft became the Windchime, a 36-foot sailboat.
In one of his last interviews, last month with The Associated Press, Schirra said he was struck by the fragility of Earth and the lack of borders.
"I left Earth three times. I found no place else to go. Please take care of Spaceship Earth," he said.
Survivors include his wife, Josephine, daughter Suzanne and son Walter Schirra III.