Boston Celtics (2015–2017)
On June 25, 2015, Mickey was selected with the 33rd overall pick in the 2015 NBA draft by the Boston Celtics. On July 20, he signed with the Celtics after playing for the team at the Utah and Las Vegas summer leagues. Over the first two and a half months of the 2015–16 season, Mickey spent the majority of his time in the NBA Development League playing for the Maine Red Claws, Boston’s affiliate team in the D-League. Between October 28 and January 18, he played three games for the Celtics and 21 games for the Red Claws.
On January 5, 2016, Mickey was named the NBA Development League Player of the Month for December 2015, becoming the first player in Red Claws franchise history to receive Player of the Month honors. Mickey led Maine to a 7–2 record (6–2 in games that he played) and averaged 19.6 points, 10.5 rebounds, 5.3 blocks and 1.8 assists per game. On December 6, he scored a season-high 32 points and grabbed 13 rebounds in a 110–97 win over the Erie BayHawks, and on December 31, he recorded a triple-double with 13 points, 13 rebounds and an NBA D-League season-high 10 blocks in a 119–112 win over the Texas Legends. On January 18, he was recalled by the Celtics after he sprained his ankle two days prior while playing for the Red Claws. On January 29, he was named in the East All-Star team for the 2016 NBA D-League All-Star Game, becoming the 13th player in Red Claws’ history to land an All-Star honor. However, he was ruled out of the All-Star game due to his left ankle injury and was replaced in the East team by Keith Benson. Mickey went on to play in seven of the Celtics’ 10 games post All-Star break, receiving limited minutes. During this stretch, he recorded a season-best three blocked shots in under two minutes of action against the Memphis Grizzlies on March 9. The following day, he returned to the Red Claws for the first time since January 18 for a one-game stint. He received three more assignments to Maine on March 13, April 5 and April 7. In 25 games (23 regular season, two playoff) for Maine in 2015–16, he averaged 17.8 points, 10.2 rebounds, 1.2 assists and 3.9 blocks per game.
Mickey appeared in eight of the Celtics’ first 19 games of the 2016–17 season. During that stretch, he scored a season-high eight points on 4-of-4 shooting in 19 minutes against the Denver Nuggets on November 6. On January 11 against Washington, Mickey made his first career start as a replacement for the injured Amir Johnson. In 14 minutes, he recorded four points, two rebounds, one assist and one block in a 117–108 win over the Wizards. During the 2016–17 season, he received multiple assignments to the Maine Red Claws, the Celtics’ D-League affiliate.
On July 13, 2017, Mickey was waived by the Celtics.
Miami Heat (2017–present)
On August 20, 2017, Mickey signed with the Miami Heat. On December 16, 2017, he scored a career-high nine points off the bench in a 90–85 win over the Los Angeles Clippers.