Marshall contributed 12 points and four rebounds in his collegiate opening game against
Morehead State. He scored a season-high 21 points in a 89–77 win over
Georgetown on February 21, 2018. Marshall was named to the
Big East All-Freshman team. As a freshman, Marshall averaged 7.7 points and 4.4 rebounds per game and made 18 starts. In the summer before his sophomore season, Marshall worked on improving his
jump shot and his conditioning. After scoring 28 points in a win at
Seton Hall on February 20, 2019, and recording 17 points in a win over
Villanova on February 24, Marshall was again named Big East player of the week. On February 28, Marshall scored a career-high 31 points as Xavier defeated
St. John's 84–73. Marshall missed a game against St. John's with an ankle injury on March 9. He was named to the Second Team All-Big East by the coaches as a sophomore. Marshall averaged 14.7 points, 7.2 rebounds, and 3.4 assists per game, shooting 39.4 percent from the field and 27.7 percent from behind the arc. After the season, he declared for the
2019 NBA draft but withdrew his name and returned to Xavier. Coming into his junior season, Marshall was named to the preseason First Team All-Big East and was on the watchlist for the
Julius Erving Award. On December 7, Marshall tied his career-high of 31 points in a 73–66 win over
Cincinnati in the
Crosstown Shootout. He sat out a game against
Western Carolina on December 18 with a stomach virus. On February 1, 2020, Marshall contributed 19 points, 10 rebounds and four assists in a 74–62 upset of tenth-ranked Seton Hall despite playing with the flu. Marshall hit the game-winning three-pointer with 4.5 seconds left in a 66–63 win at
Georgetown on March 1, finishing with 20 points and 10 rebounds. At the conclusion of the regular season, Marshall was named to the First Team All-Big East. He averaged 16.8 points, 6.3 rebounds, and 4.0 assists per game. Following the season, Marshall declared for the
2020 NBA draft. On April 9, he announced he was signing with an agent, thus forgoing his remaining season of collegiate eligibility. Marshall scored 1,277 points in three seasons as a Musketeer. ==Professional career==