On November 17, 2018, Simpson posted 14 points and 11 rebounds for his first career double-double, to help Michigan defeat George Washington 84–61. On December 30, Michigan defeated Binghamton 74–52. Simpson posted a career-high 10 assists, and one rebound shy of a double-double. On January 10, Michigan defeated Illinois 79–69. Michigan was led by Simpson with a season-high 16 points and improved to 16–0 on the season, matching the school's best start to a season. On January 13, Michigan defeated Northwestern to establish a school record for best start at 17–0 and tied the school's record 17-game win streak on the strength of a career-high 24-point effort by Simpson. On January 19, Simpson posted 10 points and 11 rebounds, for his second career double-double in a loss to Wisconsin. On January 22, Simpson was one of four Big Ten athletes named to the Naismith Defensive Player of the Year Award Top 15 midseason watchlist. On January 29 against Ohio State, Simpson posted 11 points, 10 rebounds and a career-high 12 assists, becoming the sixth player in Michigan program history to record a triple-double. The game marked Simpson's 100th career game with Michigan, becoming the 70th Wolverine to the reach the milestone. Simpson recorded his 300th career assist, becoming the 15th Wolverine to surpass the milestone. On March 3, Simpson posted 12 points and 10 assists, for his third double-double of the season in a win against (#17/#20) Maryland. Following the season, he was a 2019 Second team All-Big Ten selection (coaches and media) and a Big Ten All-Defensive Team honoree. On March 12, the U.S. Basketball Writers Association named Simpson to its 2012–13 Men's All-District V (OH, IN, IL, MI, MN, WI) Team, based upon voting from its national membership. He was named to the National Association of Basketball Coaches Division I All‐District 7 second team on March 21, as selected and voted on by member coaches of the NABC, making him eligible for the 2019 NABC Coaches’ Division I All-America team. Over the course of his junior season, he served as captain and was prolific with the hook shot, earning the nickname "Captain Hook".

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