Mack was selected by the Los Angeles Rams in the first round, as the second overall pick, in the 1966 NFL Draft. During Mack's rookie season with the Rams, starter Don Chuy was injured during the fifth game of the season, allowing Mack to move into the lineup. He started nine games during the 1966 season and became a fixture for the Rams at the left guard position for the next 13 seasons. During his NFL career, Mack never missed a game due to injury, appearing in 184 consecutive contests, the third longest streak in Rams history behind Merlin Olsen and Jack Youngblood.