Gregory Alan Hartle was born on February 14, 1951, in Savannah, Georgia.

[1][2] Hartle was selected by the St. Louis Cardinals in the tenth round, with the 251st overall pick, of the 1974 NFL draft as a linebacker.

[4] Overall, Hartle played in 14 games, starting three, for the Cardinals during his rookie year in 1974 and recovered one fumble.

[5] He appeared in 13 games, starting three in place of the injured Arneson, during the 1975 season, recording one fumble recovery and one kick return for 20 yards.

[3][6] Hartle also started the first game of the season in 1976 but was then placed on injured reserve for the rest of the year.