Weaver was born on November 1, 1928 in South Gate, California. He was the second youngest in a family of five children. Weaver briefly attended Glendale Community College but left when he was drafted into the U.S. Army. It was around this time that he met Joy Moniot, whom he married on Aug. 30, 1952, in Glendale, California.
He was a member of the L.A. County Sheriff's Pistol Team, along with Ray Chapman and several other world class shooters. In 1955, the team and individuals won the national championships at the Toledo, OH combat range using both one and two handed stances. The team defended the trophy for most of the following decade at practice matches in preparation for the National Pistol Matches, held shortly thereafter at Camp Perry, OH.
Weaver retired from the L.A. County Sheriff's department in 1979, and resided near Carson City, Nevada until his death.
The Weaver stance was developed by Jack Weaver in 1959 to compete in Jeff Cooper's "Leatherslap" matches. The stance, which incorporates a two-handed grip, isometric tension to reduce muzzle flip, and aimed fire using the weapon's sights, was adopted in 1982 as the official shooting style of the Federal Bureau of Investigation.