He was a highly decorated prep player there, earning McDonald's and Parade All-American status and was named Mr. Basketball of Arkansas in 2013.

Upon committing to Arkansas, Portis became the first in-state McDonald's All-American to sign with the Razorbacks since Corliss Williamson in 1992 and was considered a key recruit for coach Mike Anderson's rebuild of the program.

This marks the first time an Arkansas Razorback was selected since Corliss Williamson won the award back-to-back during the 1993–94 and 1994–95 seasons.

By that time he was considered one of the top players in college basketball in the 2014–15 season and a likely first-round pick in the 2015 NBA draft.

[9] With Jimmy Butler and Tony Snell struggling, Portis capitalized and had the best night of his young career with 20 points and 11 rebounds.

[30] On February 6, 2019, Portis was traded, along with Jabari Parker and a 2023 second-round pick, to the Washington Wizards in exchange for Otto Porter.

[36] On April 29, 2021, Portis scored 10 points, grabbed 11 rebounds, and recorded a career-high 4 steals in a 143–136 overtime loss to the Houston Rockets.

[37] Portis finished the regular season with the third highest three-point percentage in the NBA at 47.1%, behind Joe Harris (47.5%) and Marcus Morris Sr.

[38] During the 2021 NBA playoffs, in Game 3 of the Eastern Conference Finals on June 27, 2021, facing the Atlanta Hawks, Portis played an important role for the Bucks scoring 15 points, grabbing 4 rebounds, and adding 2 steals in a 113–102 road win.

[39] On July 1, 2021, Portis had his first career playoff start against Atlanta in the Eastern Conference Finals in place of an injured Giannis Antetokounmpo.

Portis finished with 22 points, 8 rebounds, 3 assists, and 3 steals in 36 minutes in a victory with the crowd frequently chanting "Bobby".

[40] After defeating the Hawks 4–2, the Bucks won the 2021 NBA Finals 4–2 over the Phoenix Suns, with Portis scoring 16 points in the deciding Game 6.

[48] During the first round of the 2022 NBA Playoffs, on April 22, Portis helped the Bucks to a 111–81 Game 3 win over the Chicago Bulls with 18 points and 16 rebounds.

[51] On December 3, 2022, Portis scored a game-leading 20 points, grabbed 8 rebounds, and recorded a season-high 7 assists during a 105–96 win over the Charlotte Hornets.

[58] Portis was a member of the United States national team that competed in the 2023 FIBA Basketball World Cup as a reserve.