He is a cousin of former NFL tight end Mike Cobb, whose playing career at Michigan State overlapped with Smith's at Miami. Their respective teams faced each other in the 1975 season, with Michigan State winning 14-13 which was Miami's only regular season loss that year.
Smith was drafted in the second round of the 1976 NFL Draft by the Seattle Seahawks as a running back. As a Seahawk, Smith was call "The Tank" in reference to the Sherman Tank because of his performance to run through tackles. He was the first offensive player picked by the new expansion team in its first season. He was the first Seahawk to rush for over 100 yards in a game when he ran for 124 yards against the Atlanta Falcons on November 7th, 1976.  Smith led the team in rushing in his first four seasons, eventually amassing 3,429 yards and 28 touchdowns in his seven years with the team. He also caught the ball 210 times for a total of 2,445 yards and 10 touchdowns. He played sparingly for the San Diego Chargers in the 1983 season.
The following year, Smith began teaching and coaching at Redmond Junior High School, then at Redmond High School in the state of Washington. He became the running back coach at Miami University, his alma mater, in 1990. Between 1992 and 1994, he worked as the tight end and running back coach at the University of Illinois. In 1995, Smith joined the staff of the Houston Oilers (now the Tennessee Titans) as running back coach, where he helped mold Eddie George into one of the league's elite running backs. The organization promoted Smith in 2006, giving him the title of assistant head coach in addition to his continuing duties as running back coach.
In 2008, Smith left the Titans to join the Washington Redskins as offensive coordinator, reuniting with his former Seahawks teammate, Jim Zorn, who had been named head coach of the Redskins.
After the 2009 season Zorn was fired & Mike Shanahan was named as the new head coach in Washington. Smith was not retained on staff by Shanahan. Shortly thereafter Smith was reunited with his former team, the Seattle Seahawks, when hired as a position coach (running backs).