Born and raised in Philadelphia, where rugged street basketball honed his playing style, Monroe rose to prominence at the Division II school Winston-Salem State.
In 1967 he joined the NBA team Baltimore Bullets (now the Washington Wizards), where he was Rookie of the Year in a season that included scoring 56 points in a game against the Los Angeles Lakers, the third-highest rookie total in NBA history at the time. He also spent eight years (1972-1980) with the New York Knicks, winning an NBA championship in 1973.
Monroe was named one of the 50 players on the NBA 50th Anniversary All-Time Team in 1996.
Monroe was chosen commissioner of the United States Basketball League in 1985.