After his sophomore year, he decided to transfer to The Avalon School in Wheaton, Maryland, to follow his football coach Tyree Spinner.

[6] As a junior in 2018, Diggs started the first six games of the season, before being lost for the year with a broken foot he suffered against Arkansas.

[13] Diggs was named the starter at right cornerback, helping fill the void left by Byron Jones's departure in free agency.

In Week 3 against the Seattle Seahawks, Diggs forced a fumble on DK Metcalf at the goal line and the ball broke the plane of the endzone and went out of bounds, resulting in a touchback.

[15] In Week 8 against the Philadelphia Eagles on Sunday Night Football, Diggs recorded his first two career interceptions off of passes thrown by Carson Wentz during the 23–9 loss.

[18] He started 11 out of 12 games, playing through knee and shoulder injuries, as part of a defensive unit that struggled during the season.

[23] In Week 6, Diggs had his second touchdown off an interception thrown by New England Patriots quarterback Mac Jones, giving Dallas a late 26–21 lead.

[27] Overall, Diggs finished the 2021 season with a league-leading 11 interceptions, which was also the highest single-season mark by any NFL player since Walls in 1981.

[32] Diggs registered a career-high 59 combined tackles, adding another 3 interceptions and 1 fumble recovery as the Cowboys finished 12–5 on the season.

His ACL rehabilitation was ahead of schedule and he played in the Cowboys' Week 1 matchup against the Cleveland Browns, where he recorded his first interception of the season.

[43][44] Darez Diggs played for Friendship Collegiate Academy Public Charter School in Washington, D.C., the UAB Blazers,[45] and the Morgan State Bears.