[3] Greenlaw started at cornerback, recording 53 total tackles, two forced fumbles, five pass breakups, and an interception on a team that repeated as 7A state champions.

[6][7] Greenlaw played college football under head coaches Bret Bielema and Chad Morris at Arkansas.

[13] Greenlaw made his NFL debut in the season-opening 31–17 road victory over the Tampa Bay Buccaneers, recording four tackles.

[15] In the next game against the Seattle Seahawks on Monday Night Football, Greenlaw recorded eight tackles, a pass deflection, and his first NFL interception off of Russell Wilson in a 27–24 overtime loss.

[19] In the regular-season finale against the Seahawks on Sunday Night Football, Greenlaw once again recorded a team-high 13 tackles and made a critical stop on tight end Jacob Hollister inside of the one-yard line on the final play of the drive, sealing a 26–21 road victory and securing the first seed for the 49ers in the 2019 playoffs.

[20] Greenlaw credited review from the 49ers' Week 15 29–22 loss to the Atlanta Falcons, which ended in a similar scenario where wide receiver Julio Jones was barely able to break the plane into the endzone, for giving him the correct technique to hit Hollister high.

[21][22] Greenlaw finished his rookie year with 92 tackles, two pass deflections, a sack, and an interception in 16 games and 11 starts.

[26] During the National Football Conference (NFC) Championship Game against the Green Bay Packers, he recorded six tackles as the 49ers won by a score of 37–20.

[30] In the next game against the New England Patriots, Greenlaw recorded six tackles and his first sack of the season on Cam Newton as the 49ers won on the road by a score of 33–6.

[33] During the season-opening 41–33 road victory over the Detroit Lions, Greenlaw recorded six tackles and his first defensive touchdown on a pick six off of Jared Goff.

[49] Three weeks later against the Miami Dolphins, he recorded a team-high eight tackles, a pass deflection, and recovered a Tua Tagovailoa fumble forced by Nick Bosa and returned it 23 yards for a touchdown to secure a 33–17 victory for the 49ers.

[50] In the next game against the Buccaneers, Greenlaw recorded a team-high 15 tackles, a pass deflection, and intercepted Tom Brady once in the 35–7 victory.

[54] The 49ers defeated the Seahawks in the Wild Card Round by a score of 41–23 behind a strong performance by Greenlaw, who recorded a team-high 11 tackles.

[60] During Super Bowl LVIII against the Kansas City Chiefs, Greenlaw fell over after coming off the sideline, which turned out to be a left leg injury.