Leno made the decision when he was told the Shooting, Hunting, Outdoor Trade Show is hosted by the National Shooting Sports Foundation, a group that lobbies for gun owners and manufacturers, said Dick Guttman, spokesman for the former NBC "Tonight Show" host.
"He thought it was a sporting group," Guttman said. "He understands that it caused concern. He fully understands the sensitivities."
A statement by the foundation expressed disappointment that Leno won't perform at the 2015 SHOT Show industry dinner in January.
The statement blamed the decision on pressure from an anti-gun lobby and "false statements about our industry and its commitment to genuine firearms safety."
Foundation spokesman Bill Brassard didn't immediately respond to messages seeking additional comment.
Newtown Action Alliance leader Po Murray said Leno called her Wednesday, just hours after her group announced it had collected 6,000 signatures urging Leno to cancel.
Murray and Guttman both said Leno characterized the booking as a mistake.
Murray noted it has been less than two years since a rampaging gunman killed 20 children and six staff members in December 2012 at Sandy Hook Elementary School in Newtown, Connecticut. The gunman, 20-year-old Adam Lanza, also killed himself.
The massacre led to the formation of Murray's group, a non-profit led by volunteers that advocates for stricter gun laws.
"To think a celebrity of that stature would participate in raising funds for the gun lobby was extremely disheartening for us," Murray said, referring to Leno.
The National Shooting Sports Foundation has been based in Newtown for many years. Its SHOT Show is for trade members only and is not open to the general public.
The show will be held Jan. 20-23 at the Sands Expo and Convention Center. Organizers say it drew 1,600 exhibitors and 70,000 conferees in 2014.