Simmons and Mitchell received the maximum 100 first-team votes, while the Boston Celtics' Jayson Tatum received 99. Los Angeles Lakers forward Kyle Kuzma and Chicago Bulls forward Lauri Markkanen rounded out the first-team selections.
"Somebody made a mistake," Celtics coach Brad Stevens told reporters of Tatum's not being a unanimous selection.
Dallas Mavericks guard Dennis Smith Jr., Lakers guard Lonzo Ball, Atlanta Hawks forward John Collins, Sacramento Kings guard Bogdan Bogdanovic and Phoenix Suns forward Josh Jackson make up the All-Rookie second team.
Philadelphia guard Markelle Fultz, the No. 1 pick in last June's draft, did not make an All-Rookie team after missing most of the season due to a shoulder injury. It's the second straight season the top overall selection was left off the ballots; Simmons, the 2016 No. 1 pick, sat out all last season with a broken foot.
The last top overall pick to not make an All-Rookie team due to factors other than injury was the Cleveland Cavaliers' Anthony Bennett in 2013-14.
Simmons and Mitchell are the runaway favorites for Rookie of the Year, and the former looks like the likely winner. Simmons averaged 15.8 points, 8.2 assists and 8.1 rebounds per game during the regular season; he and Oscar Robertson in 1960-61 are the only players to put up a 15-8-8 stat line during their first year.
Mitchell averaged 20.5 points, 3.7 assists and 3.7 rebounds during the regular season. He was the first rookie who did not first sit out a season to average 20 points per game since Tyreke Evans with Sacramento in 2009-10.
Most expected Ball and Kuzma to switch places on their respective teams when drafted, but Kuzma has been arguably the steal of the entire draft. He averaged 16.1 points and 6.3 rebounds this season.
"That's one of my goals I had at the beginning of the year," Kuzma told reporters of making the All-Rookie team, adding, "To see it come true is pretty rewarding."