As a young man, Windham played football at Texas Western College, now known as the University of Texas at El Paso. He then went on to play for the New York Jets during the 1966 pre-season and received tryouts with the New Orleans Saints and Denver Broncos. After football, at the urging of Wahoo McDaniel, Windham trained with Joe Blanchard in Corpus Christi, Texas and later with Verne Gagne and became a professional wrestler in the American Wrestling Association. He then moved to the World Wide Wrestling Federation and became the villain Blackjack Mulligan. At first Mulligan, donning black trunks, black hat, black glove, and using the iron claw submission hold, seemed to be an imitation of AWA wrestler Blackjack Lanza, managed by The Grand Wizard. Mulligan went on to greater success in the Northeast. He received a push and went on to wrestle Pedro Morales and Bruno Sammartino.
Mulligan returned to the Midwest and tagged with Lanza to form The Blackjacks. The duo went on to capture numerous tag team championships in various NWA affiliated promotions as well as the WWWF World Tag Team Championship in August 1975.
Mulligan often battled Andre the Giant, feuding in many different regions in the early 1980s. When they brought their feud to the WWF in 1982, Windham was noted as saying Andre had no limit to his strength. Mulligan would go onto wrestle in Florida as a face. He often teamed with West Texas stars Dusty Rhodes, Dick Murdoch, and his son Barry Windham.
In 1986, Mulligan wrestled under a mask as "Big Machine," part of a team with "The Giant Machine" (Andre the Giant) and "Super Machine" (Bill Eadie) collectively known as The Machines. The Machines were wildly popular, winning several high-profile matches against the Heenan family, recruiting members such as Hulk Machine and Piper Machine; the angle was ten years ahead of its time, being comparable to the NWO angle of the 90's. Later on, Jack travelled to Dallas and competed in World Class Championship Wrestling, as a heel, wrestling against Bruiser Brody, Chris Adams and Kevin and Lance Von Erich.
Mulligan and his son Kendall were in trouble with the law for counterfeiting charges in 1989 and were sentenced to jail-time.