He played college football for the Oregon Ducks, and was selected by the San Francisco 49ers in the first round of the 2016 NFL draft.

[8] Buckner was voted the 2015 Pac-12 Conference Defensive Player of the Year by the Pac-12 coaches, joining Haloti Ngata as the second Oregon Duck to ever win the award.

Buckner made his NFL debut in the season-opener against the Los Angeles Rams, recording one tackle in the 28–0 shutout victory.

[17] During Week 14 against the New York Jets, Buckner recorded a team-high 11 tackles and sacked Bryce Petty twice in the 23–17 overtime loss.

[18] Buckner finished his rookie season with 73 tackles, six sacks, two fumble recoveries, and a pass deflection in 15 games and starts.

[21] Three weeks later against the Arizona Cardinals, Buckner recorded four tackles and his first sack of the year on Carson Palmer during the 18–15 overtime road loss.

[22] During Week 15 against the Tennessee Titans, Buckner recorded a season-high nine tackles and half a sack on former college teammate Marcus Mariota in a narrow 25–23 victory.

[23] Buckner finished his second professional season with 61 tackles, five pass deflections, three sacks, and a forced fumble in 16 games and starts.

[24] During the season-opener against the Minnesota Vikings, Buckner recorded seven tackles and sacked Kirk Cousins 2.5 times in the 24–16 road loss.

[31] During Week 2 against the Cincinnati Bengals, Buckner recorded a tackle and his first sack of the season on Andy Dalton in the 41–17 road victory.

[32] In the next game against the Pittsburgh Steelers, Buckner recorded a season-high 10 tackles, sacked Mason Rudolph once, and recovered a fumble forced by teammate Arik Armstead on James Conner late in the fourth quarter to help seal a 24–20 victory.

In that game, Buckner recorded five tackles and a strip sack on Baker Mayfield which was recovered by teammate Nick Bosa as the 49ers won by a score of 31–3.

[35] In the next game against the Seahawks on Monday Night Football, he recorded five tackles and recovered a fumble forced by teammate K'Waun Williams on running back Rashaad Penny.

Later in the game, Buckner recovered a fumble lost by offensive tackle Germain Ifedi and returned it for a 12-yard touchdown during the 27–24 overtime loss.

[40] During the NFC Championship Game against the Green Bay Packers, Buckner recorded four tackles and recovered a fumble lost by Aaron Rodgers in the 37–20 victory.

[41] During Super Bowl LIV against the Kansas City Chiefs, Buckner recorded six tackles and sacked Patrick Mahomes 1.5 times in the 31–20 loss.

[46] During Week 11 against the Packers, Buckner recovered a fumble forced by teammate Julian Blackmon on wide receiver Marquez Valdes-Scantling early in overtime to help the Colts win the game 34–31.