He played college football for the South Carolina State Bulldogs and was selected in the second round of the 2018 NFL draft by the Indianapolis Colts.

He posted 70 stops the following year (2015), 131⁄2 of those for loss, with five sacks and two interceptions to earn a first-team all-conference selection as a redshirt sophomore.

Leonard saved his best for last, as he posted career highs in total tackles (113) and sacks (81⁄2) as a senior while picking off two more passes and forcing one fumble.

[7] On January 27, 2018, Leonard played in the 2018 Senior Bowl and recorded a game-high 14 combined tackles (five solo) as part of Bill O'Brien's North team that defeated the South 45–16.

On March 20, 2018, Leonard participated at South Carolina State's pro day and performed the rest of his combine drills.

At the conclusion of the pre-draft process, Leonard was projected to be a second or third round pick by NFL draft experts and scouts.

[23] On November 18, 2018, Leonard made seven combined tackles, one pass deflection, one sack, and his first career interception during a 38–10 victory against the Tennessee Titans in Week 11.

Leonard intercepted a pass by Titans’ backup quarterback Blaine Gabbert, that was intended for wide receiver Tajae Sharpe, and returned it for a ten-yard gain during the third quarter.

[24] In Week 15, Leonard broke the Colts' franchise record for most tackles by a rookie in a single season with 143, in a 23–0 win vs the Dallas Cowboys.

In Week 17, Leonard broke the Colts’ franchise record for most tackles in a single season with 163, in a 33–17 win vs the Tennessee Titans.

[26] He finished his rookie season in 2018 with 163 combined tackles (111 solo), eight pass deflections, seven sacks, and two interceptions in 15 games and 15 starts.

[30] On January 5, 2019, Leonard started in his first career playoff game and recorded 13 combined tackles (five solo) and broke up a pass attempt during a 21–7 victory at the Houston Texans in the AFC Wild Card Round.

[37] In Week 14, against the Tampa Bay Buccaneers, Leonard recorded nine tackles and intercepted two passes thrown by Jameis Winston, one of which was returned for an 80 yard pick six, during the 38–35 loss.

[42] He additionally in 2019 became the eighth player in NFL history to record five sacks and five interceptions in a single season, joining the likes of Pro Football Hall of Famers Brian Urlacher, Wilber Marshall and LeRoy Butler; Leonard was the first to achieve the feat in fewer than 15 games.

[45] In Week 9 against the Baltimore Ravens, Leonard recorded a team high 15 tackles (13 solo) and recovered a fumble forced by DeForest Buckner on Gus Edwards during the 24–10 loss.

[47] In Week 15 against the Texans, Leonard led the team with 15 tackles and forced a fumble on wide receiver Keke Coutee that was recovered by the Colts during the 27–20 win.

[48] In Week 17 against the Jacksonville Jaguars, Leonard led the team with 10 tackles and recorded a strip sack on Mike Glennon that was recovered by the Colts during the 28–14 win.

After a November 11, 2017 game in which South Carolina State defeated Hampton, Leonard proposed to his girlfriend, Kayla, whom he had known since kindergarten.