He played college football for the Notre Dame Fighting Irish and was selected 34th overall by the Dallas Cowboys in the second round of the 2016 NFL draft.

Smith attended Bishop Luers High School in Fort Wayne, Indiana, where he was a letterman in football, basketball and track.

[4] Smith signed his letter of intent on February 6, 2013, and committed to attend and play football at the University of Notre Dame.

Sources: In 2013, Smith won the starting DOG linebacker position beating out Ben Councell, after incumbent starter Danny Spond had to retire due to migraine issues.

[10] He was one of three players to start all 13 games on defense, recording 112 tackles, including nine for loss, and 3.5 sacks, adding two pass breakups and one forced fumble.

[17] On January 11, 2016, Smith released a statement through Twitter announcing his decision to forgo his remaining eligibility and enter the 2016 NFL Draft.

[18] Smith attended the NFL Scouting Combine in Indianapolis in order to meet with teams, but was unable to participate in workouts due to his injury.

[27][28] Jared Dubin, a CBS Sports writer, stated in an article posted to CBSSports.com on June 29, 2016, that "The Cowboys are confident in their diagnosis because it was their team physician that did Smith's surgery."

[31] On July 20, 2017, the Dallas Cowboys signed linebacker Justin Durant as insurance in the event Smith was unable to fully recover before the start of the regular season.

[33] The Dallas Cowboys planned to use Smith as a backup and rotational player primarily on first and second down to avoid putting him in pass situations.

[35] He made his professional regular season debut and first career start in the Dallas Cowboys’ season-opener against the New York Giants and recorded seven combined tackles and forced a fumble during their 19–3 victory.

[34] On October 22, 2017, Smith recorded three combined tackles and forced a fumble on his first career sack during the Cowboys’ 40–10 win at the San Francisco 49ers in Week 7.

Smith made his first career sack on 49ers’ quarterback C. J. Beathard for a six-yard loss and also forced a fumble by Bethard that was recovered by teammate Tyrone Crawford in the third quarter.

[37] Smith finished his 2nd NFL season in 2017 with 81 combined tackles (50 solo), two pass deflections, two forced fumbles, and one sack in 16 games and six starts.

[38] Smith entered training camp slated as the starting middle linebacker after Anthony Hitchens departed for the Kansas City Chiefs during free agency.

[39] On October 7, 2018, Smith collected 12 combined tackles (six solo), broke up a pass, and made a sack during a 19–16 loss at the Houston Texans in Week 5.

In week 9 against the New York Giants on Monday Night Football, Smith recorded a team high 12 tackles and half a sack on Daniel Jones in the 37–18 win.

[45] In week 10 against the Minnesota Vikings on Sunday Night Football, Smith recorded a team high 13 tackles in the 28–24 loss.

[46] In week 17 against the Washington Redskins, Smith recorded a team high eight tackles and intercepted a pass thrown by Case Keenum during the 47–16 win.

It was reported in the media, that the change forced Smith to pay a mid-six figure sum, in order to buy back the existing inventory of jerseys and T-shirts with his old number.

[55] On October 5, 2021, the Cowboys released Smith after he declined to waive his 2022 injury guarantee contract clause, to protect the team from a possible payment of $9.2 million, and after he was passed on the depth chart by rookie Micah Parsons and veteran Keanu Neal.

He appeared in 13 games with 11 starts, tallying 74 tackles, one sack and one fumble recovery,[67] while leading the linebackers with 724 defensive snaps (including the playoffs).