Thompson attended Los Angeles High School and Los Angeles City College before transferring to UCLA in 1948. He started at guard for the Bruins for two seasons, 1948 and '49, joining the Rams in 1950.
Thompson was a key blocker for the Rams' renowned "Bull Elephant" backfield that featured Dan Towler, Dick Hoerner and Paul "Tank" Younger.
In the 1951 championship game, Thompson's blocking was key in the Rams' 24-17 victory over Cleveland.
"We came out of that [Coliseum] tunnel so sky-high, I don't think our feet touched the ground," Thompson told The Times in 1991. "I don't think I've ever seen a team as ready to hit as we were."
Thompson was with the Rams from 1950 to 1954 and finished his career playing for the Chicago Cardinals in 1955.
"Harry was one of those self-sacrificing players that every good team has on the roster," said retired Times sportswriter Bob Oates, who covered Thompson when the athlete was a Ram and remained a close friend. "Harry was always the next-to-last cut in training camp, but he never got cut, because he was always too good."
As one of the first African American players in the National Football League, Thompson also endured the racism of the era.
"In Washington, D.C., ... we stayed in this hotel and we could sleep in the hotel. But we couldn't eat in the hotel restaurant," Thompson once noted, recalling that the black players on the Rams were allowed only to order room service.
After he retired as a player, Thompson became an activist for pension benefits for those who had played professional football before 1959. He spent a year as president of NFL Alumni Inc. and once collected 1,600 signatures on petitions to get more money for pre-1959 players.
"He always pushed the league to do more for old-time players," Oates said of Thompson, who attended every Super Bowl game. "He made it a point to try to get the league to raise its pension for [pre-1959] players."
Thompson is survived by daughters Brenda Joyce Cyrus-Chambers and Jetawyn McDonald, and sons Andrew Cyrus and Harry Thompson Jr.
A funeral Mass will be celebrated at 11 a.m. Wednesday at St. Jerome Catholic Church in Westchester. More information is available at (310) 936-2745.