A left-hander, Humm was selected by the Oakland Raiders in the fifth round of the 1975 NFL Draft, the 128th overall pick. The next year, he was a part of the Super Bowl XI championship team. He was the backup to Ken Stabler for five seasons and was also an effective holder for a field goals or extra points, with the unusual habit of arriving on the field with no shoulder pads, which tipped the opposing team that a fake field goal was unlikely.
In 1981, he signed as a free agent with the Baltimore Colts. That year, the game between the Colts and the Dallas Cowboys was the only start in his NFL career when injuries sidelined Bert Jones and Greg Landry, in which he completed 7 of 24 passes for 90 yards and 2 interceptions, in a 13–37 loss, the thirteenth consecutive for the Colts. Notably, the opposing quarterback for the Cowboys, Glenn Carano, was also making his first and only NFL start in place of Danny White, the only time in league history two "one and done" quarterbacks have ever faced off. Carano and Humm were the subjects of an NFL Films piece entitled "My One and Only," recounting the 1981 game. The two quarterbacks had been friends since high school (both played high school football in Nevada), and remained friends until Humm's death.
Humm held the distinction of being the only Raiders player that was a member of both the Raiders' 1976 Super Bowl XI and 1983 Super Bowl XVIII championship teams, but not the 1980 Super Bowl XV championship team in between the other two.
In 1988, Humm was diagnosed with multiple sclerosis at age 36 and lost the use of his legs in 1997. He had set up a broadcasting studio in his home and worked as a color commentator for the Oakland Raiders. Humm died due to complications from multiple sclerosis on March 27, 2018.