Hartenstine was born on July 27, 1953, in Bethlehem, Pennsylvania, in the Lehigh Valley region of eastern Pennsylvania, where he played high school football at Liberty High School in Bethlehem.
He played college football at Pennsylvania State University, where he was a collegiate All-American.
Hartenstine entered the 1975 NFL draft and was selected by the Chicago Bears in the second round with the 31st overall pick.
In his 1975 rookie season with the Bears, he was named to the Pro Football Writers of America NFL All-Rookie Team.
Hartenstine was the oldest player on the Super Bowl XX-winning Bears team in 1985.