With Elena Delle Donne approaching restricted free agent status and her rookie contract expiring in the 2017 offseason, she was traded to the Washington Mystics in exchange for Stefanie Dolson, Kahleah Copper and the second overall pick in the 2017 WNBA draft just a couple days before the start of free agency.
On May 14, 2017, Elena Delle Donne made her debut for the Washington Mystics, scoring a team-high 26 points in an 89–74 win over the San Antonio Stars. On May 19, 2017, Delle Donne scored a season-high 27 points in a 99–89 loss to the Los Angeles Sparks. On September 1, 2017, Delle Donne scored a new season-high of 37 points along with a career-high 6 three-pointers in a 110–106 overtime win against the Seattle Storm as Washington Mystics secured a playoff berth as the No. 6 seed in the league with an 18–16 record.
In the 2018 season, with Meesseman sitting out the season to play for Team Belgium in the FIBA World Tournament, Elena Delle Donne played at the power forward position in the starting lineup. On June 13, 2018, Elena Delle Donne scored a season-high 36 points in a 95–91 victory over the Connecticut Sun. After scoring 23 points in a 77–80 loss to the Atlanta Dream, Elena Delle Donne became the fastest player in WNBA history to score 3,000 points, completing the feat in 148 games. Delle Donne was voted into the 2018 WNBA All-Star Game, receiving her fifth all-star appearance. By the end of regular season, Elena Delle Donne averaged 20.7 ppg. Washington Mystics were the number three seed with a 22–12 record, receiving a bye to the second round elimination game.
During the 2019 season, Elena Delle Donne was voted into the WNBA all-star game, making it her 6th all-star appearance and was also named Team Captain after receiving the top two votes among all-stars in the league for the new all-star game voting format. On July 30, 2019, she scored a season-high 33 points in a 99–93 victory over the Phoenix Mercury. By the end of the season and with the help of Meesseman’s return, the Mystics finished as the No. 1 seed in the league with a 26–8 record, receiving a double-bye to the semi-finals. During the playoffs it was announced that Elena Delle Donne had won the MVP award, making it her second career MVP award. She also made the All-WNBA First Team and finished the regular season with a new career-high in field goal percentage. In the semi-finals, the Mystics defeated the Las Vegas Aces 3–1 to advance to the WNBA Finals for the second year in a row, also making it Elena Delle Donne’s third finals appearance and later earned her first WNBA championship. She also became the first woman to join the prestigious 50–40–90 club (50% field goals, 40% 3-point shooting and 90% free throws over a single season), marking great all-around shooters.
In 2020, Elena Delle Donne opted to sit out the entire season due to health concerns and preexisting risk factors with the COVID-19 pandemic. Elena Delle Donne was limited to three games during the 2021 WNBA season. During that season, she was named to The W25 as one of the top 25 players of the league’s first 25 years.
In February 2024, Elena Delle Donne announced that she would step away from basketball and did not sign the contract Washington Mystics had offered her.
