The son of former NHL defenseman Per Djoos was a regular for Brynas in the Swedish Hockey League at age 19. On May 16, 2014, he signed a three-year contract with the Washington Capitals, who had selected him in the seventh round (No. 195) of the 2012 NHL Draft.

Djoos spent two full seasons with Hershey, Washington's affiliate in the American Hockey League. He went from 22 points (eight goals, 14 assists) in 2015-16 to 58 points (13 goals, 45 assists) in 2016-17.

He made Washington's opening-night roster in 2017-18 and became the first defenseman in Capitals history to have two points in his NHL debut when he had a goal and an assist against the Pittsburgh Penguins on Oct. 11, 2017. Djoos finished the season with 14 points (three goals, 11 assists) in 63 games, then played in 22 games during the 2018 Stanley Cup Playoffs to help the Capitals win their first championship since entering the NHL in 1974. He continues to show the kind of maturity that should help him earn a top-four role in the not-too-distant future.