He was 41.
Mirra's body was found in a truck in his hometown of Greenville, North Carolina, about 4 p.m. Thursday, shortly after he visited friends in the area, Greenville police said.
When Mirra retired from BMX competition in 2011, he had 24 X Games medals, the most in the history of the ESPN-run extreme sports competition.
That total, which included one medal from rally car racing, the discipline he focused on after BMX retirement, was surpassed in 2013 by skateboarder Bob Burnquist.
"We mourn the loss today of a great friend and wonderful human being who touched the lives of so many around the world with his gift," Greenville Mayor Allen Thomas said in a news release. "He called Greenville, North Carolina, home and was as humble a guy talking with kids on a street corner about bikes as he was in his element on the world stage.
"A young life with so much to offer was taken too soon."
Hours before his body was found, photos were posted to his Instagram account. One showed him and his wife, ESPN reported. Another featured boxing, a sport he had taken up as an amateur.
"Fight to win! We all have a battles to fight. Never back down. Love you all. #diewithyourbootson," the caption read.
Skateboarding star Tony Hawk, reacting to Mirra's death on Twitter, called him a pioneer, icon and legend.
"Thank you for the memories ... we are heartbroken," he tweeted.
Mirra turned pro as a BMX rider in 1992 and competed in the inaugural X Games in Rhode Island in 1995. He medaled in each of his first 12 X Games competition starts, from 1995 to 2000, according to ESPN.
He won five straight X Games golds in BMX Park -- a freestyle riding competition in a skateboard-park setting -- from 1996 to 2000, and three straight golds in BMX Vert, on a halfpipe, from 1997 to 1999.
In a 2013 interview, he told XGames.com
that he retired from BMX in part because of injuries.
"It was never going to be that fun for me to go on riding on a plateau level and not keep progressing, but by the same token I got to a point where I really couldn't take getting injured anymore in the name of progression," Mirra said. "It put me in a stalemate. I probably could have milked BMX for a lot longer if I'd really wanted to, but that's not my style."
He is survived by his wife and two children, police said.