He died in his sleep at his home in Sequim, Wash., early Tuesday, the Rangers said. He was being treated for esophageal cancer.
Keller spent most of his baseball career in scouting and the front office and was the first farm director of the expansion Washington Senators.
After the Senators moved to Texas and became the Rangers, Keller introduced the radar gun, which measures the speed of pitches. The Rangers said Keller did so on the recommendation of former major league outfielder and Michigan State coach Danny Litwhiler.
Keller was the younger brother of the late Yankees standout Charlie "King Kong" Keller, a five-time All-Star who played in same outfield with Joe DiMaggio. Hal Keller played 25 games with Washington from 1949-52.
Keller was farm director of the Senators from 1961-62 and 1965-71 and then held the same role from 1972-78 with the Rangers. He joined Seattle's front office from 1979-85, first as director of player development before becoming general manager for three years.
He later scouted for the Angels, Detroit Tigers and Cleveland Indians.
Memorial services will be held in his native Maryland and in Washington state in the next few weeks.