Smart was born in Montgomery, Alabama but grew up in Bainbridge, Georgia. The son of a high school football coach, Smart began his playing career at Bainbridge High School and went on to play college football at the University of Georgia, where he was teammates with defensive linemen Antonio Cochran, Emanos Leroy, linebacker Brandon Tolbert, and future All-Pro cornerback Champ Bailey. Smart was a four-year letterman at defensive back for Georgia and a first-team All-SEC selection as a senior. He finished his career with 13 interceptions, which ranks fourth all-time at Georgia, and led the Bulldogs with six interceptions in 1997 and five in 1998. He also was a four-time member of the SEC Academic Honor Roll. Smart graduated from Georgia in 1999 with a degree in finance. He went undrafted in the 1999 NFL Draft and signed a free agent contract with the Indianapolis Colts. He spent the 1999 preseason with the team but was cut before the start of the regular season.
Smart began his coaching career with the University of Georgia in 1999, serving as an administrative assistant. He then moved to Valdosta State where he spent one season as defensive backs coach before being promoted to defensive coordinator for the 2001 season. From 2002–2003 , Smart worked as a graduate assistant under Bobby Bowden at Florida State while pursuing a master's degree. He received his master's degree from FSU in 2003. Smart then spent one season as defensive backs coach at LSU under head coach Nick Saban in 2004. Smart rejoined the Georgia Bulldogs football program to serve as running backs coach for the 2005 season. His only season in the NFL came in 2006, during which time he coached under Nick Saban again, this time as the Miami Dolphinssafeties coach.
Smart followed Nick Saban to the University of Alabama in 2007. He was hired by Saban as an assistant coach on January 9. On February 27, 2008, Smart was promoted to defensive coordinator. On December 8, 2009, Smart was awarded the Broyles Award as the nation's best assistant coach. He is the first Alabama assistant coach to win the award. Smart considered a lucrative contract to be the defensive coordinator at his alma mater, the University of Georgia, but chose to stay with the Tide in early January 2010. On March 27, 2012, the University of Alabama System's Board Of Trustees voted to increase Smart's salary and extend his contract. Smart remains the highest-paid football assistant on head coach Nick Saban's staff. On November 20, 2012, Smart was recognized as the 2012 AFCA FBS Assistant Coach of the Year. On April 16, 2013, Smart was granted a $200,000 salary increase to make him the highest-paid defensive coordinator in college football.
Smart was a member of the Georgia Beta chapter of the Sigma Alpha Epsilon fraternity. He is married to Mary Beth Lycett, who played basketball for the University of Georgia. They have three children.