Miller, a former Major League Baseball Players Association leader, revolutionized the sport. He led baseball is establishing free agency for players after six seasons of service, salary arbitration and grievance arbitration. He also was the union leader through five work stoppages.
Miller enters the Hall of Fame after failing to collect enough veterans committee votes in his first seven attempts for the honor. He died in 2012 at 95 years old. Miller was the union leader from 1966 to 1982.
"Players are pleased that Marvin will now take his rightful and long overdue place in the Hall of Fame in recognition of the monumental and positive impact he had on our game and our industry," MLBPA executive director Tony Clark said in a statement.
Simmons, 70, was an eight-time All-Star. He played 13 seasons with the St. Louis Cardinals. The switch-hitting catcher also spent time with the Atlanta Braves and Milwaukee Brewers during his 21 seasons.
Simmons was named on 13 of 16 ballots, achieving 81.3 percent of the vote. Miller earned 75 percent of the vote, the exact amount needed for entry into the Hall of Fame.
"If it weren't for the analytics people, my career as a potential Hall of Famer probably would have been shut down and forgotten a long time ago," Simmons told MLB.com. "When they started talking about on-base percentage and WAR and how WAR was comprised, it became a real study and then the real comparisons started to develop."
Miller and Simmons will be inducted during ceremonies July 26 in Cooperstown, N.Y. Players for this year's inductions will be chosen in January by the Baseball Writers' Association.