Wagner was selected by the Pittsburgh Steelers in the 11th round of the 1971 NFL Draft, the 268th overall selection. First looked at as a wide receiver on offense, he was soon switched to defense as a safety. Wagner tied for the NFL lead with eight interceptions in 1973, appeared in the 1975 and 1976 Pro Bowls and recorded 36 career interceptions and 12 fumble recoveries. He won four Super Bowls, recording interceptions in Super Bowl IX and Super Bowl X. After ten seasons with the Steelers, he retired in January 1981.
For a short time, Mike Wagner also served as an assistant coach on the Pine-Richland High School (Gibsonia, PA) coaching staff, mainly working with the defensive backs and special teams.
Mike Wagner was the guest speaker at the 3rd annual Steel City Mafia Banquet in Latrobe, Pennsylvania, on August 15, 2009.
After pro football, Wagner applied his bachelor's degree in accounting and worked in the financial industry. He earned an M.B.A. from the University of Pittsburgh and, as of 2012, is a bank vice president in Pittsburgh.