Mansfield moved to the Steelers in 1964 and played left defensive tackle for two years. He switched to offense and was their starting center for a decade, from 1966 until 1976, and was a key member of the Steelers' Super Bowl-winning teams of the 1974 and 1975 seasons. In his last season as a Steeler, he kicked the extra point on the Steelers' final touchdown in a playoff game at Baltimore, after Roy Gerela pulled a groin muscle. Mansfield was also a placekicker in college and for three college all-star teams.
Mansfield played in 182 consecutive games and was succeeded at center by Mike Webster. He retired from pro football in July 1977, and went on to a successful career as an insurance broker in the Pittsburgh area, previously his off-season job.
Known by his Steeler teammates as "Ranger" for his love of the outdoors, Mansfield died at age 55 from a heart attack while hiking with his son in the Grand Canyon in Arizona in November 1996. He was buried at Desert Lawn Memorial Park in his hometown of Kennewick.