After earning co-Most Valuable Player honors for the district as a senior at M. B. Smiley High School of Houston in 2007–08, Simmons spent his college freshman season in 2008–09 playing for Paris Junior College, where he averaged 12.2 points and three rebounds per game, and earned all-conference honors.

For his sophomore season, Simmons transferred to Midland College, where he was an all-conference and all-region selection in 2009–10, after averaging 13.3 points and four rebounds per game.

[1] On November 15, 2010, Simmons signed a National Letter of Intent to play college basketball for the University of Houston.

[1] As a junior at Houston in 2011–12, Simmons was named to the RAMADA All-College Classic All-Tournament Team and earned Conference USA Player of the Week honors on February 20, 2012.

[6] In September 2013, Simmons tried out for the Austin Toros of the NBA Development League[7] and made the team's training camp roster.

[12] During his time in Austin, Simmons considered giving up on his basketball career, moving back home and working a 9-to-5 job to take care of his four daughters.

[14] On June 29, 2015, Simmons was named in the Brooklyn Nets' Summer League roster to compete in Orlando and Las Vegas.

[32] In the Spurs' regular season finale on April 13, Simmons scored a then career-high 19 points in a 96–91 win over the Dallas Mavericks.

[34] In the Spurs' season opener on October 25, 2016, Simmons scored a career-high 20 points in a 129–100 win over the Golden State Warriors.

[37] On May 11, 2017, Simmons made his first career playoff start, filling in for the injured Kawhi Leonard and scoring 18 points in a 114–75 win over the Houston Rockets in Game 6 of their second-round series.

[48] On June 21, 2019, Simmons was traded, along with the draft rights to Admiral Schofield, to the Washington Wizards in exchange for cash considerations.

[50] On February 25, 2020, the Santa Cruz Warriors announced that they had acquired Simmons and the returning right to Chris Walker from the Rio Grande Valley Vipers in exchange for returning rights to both Antonius Cleveland and Jared Cunningham plus a first round draft pick in the 2020 NBA G League draft.

[51] On March 5, 2020, Simmons posted his third straight game with at least 18 points in addition to six rebounds in a loss against the Salt Lake City Stars.