Smith attended Knoxville Catholic High School in Knoxville, Tennessee. In his senior season, he gained 1,340 yards rushing to go with 19 touchdowns, caught 23 passes for 453 yards and six TDs, and also made 61 tackles, two tackles for loss, two interceptions and caused two fumbles on defense. He was named Gatorade Football Player of the Year in Tennessee in 2006.
Considered a four-star recruit by Rivals.com, he was listed as the No.25 Athlete in the nation. Many scouts believed he could play offense or defense in college. He chose Notre Dame over offers from Tennessee, Auburn, and Alabama.
After red-shirting as a freshman in 2007, he emerged as a star in 2008. He began his career at linebacker. In his first year, he started 9 games and recorded 57 tackles, which included 8.5 tackles for loss, and 3.5 sacks. As a junior in 2009, he started 6 games at strong safety, before moving to strongside linebacker for the final 6 games. He recorded 69 tackles including 6.5 tackles for loss. In 2010, as a senior, he started all 13 games for the Irish at strong safety where he recorded 93 tackles including a team-high 7 interceptions. In 2011, he was accepted to a graduate program at Notre Dame allowing him to play another season. In his final year in 2011, serving as team captain, he played in and started in all 13 games. He recorded 90 tackles including 3 tackles for loss.
Smith was drafted by the Minnesota Vikings in the first round, 29th overall in the 2012 NFL Draft. On October 8, 2012, Smith was ejected after flailing against an official. On October 19, 2012, he was fined $15,750 for a horse-collar tackle on Robert Griffin III. On October 21, 2012, Smith had his first career interception and returned it for a touchdown against the Arizona Cardinals. On December 9, 2012, Smith had his third career interception and returned it for a touchdown against the Chicago Bears.