In 2014, Allen was selected as a McDonald's All-American out of Providence School in Jacksonville, Florida, where he won a state championship the previous year.

[5] He won the McDonald's All-American Slam Dunk Contest, jumping over future Duke teammate Jahlil Okafor.

[6] During Allen's freshman season at Duke, he averaged 4.4 points per game and was named to the ACC all-academic team.

[11][12] In Duke's second game against Louisville, Allen exchanged elbows while scuffling for the ball and was later ejected for yelling at a referee after being called for his fifth foul.

[13] On February 25, in the closing seconds of a 15-point win against Florida State, Allen tripped opposing guard Xavier Rathan-Mayes.

[14] A day later, Allen received a reprimand from the Atlantic Coast Conference (ACC) for his second tripping incident in less than a month.

[15] During the 2015–16 season, Allen was one of the best offensive players in the ACC, averaging 21 points per game[16] and shooting 41% from three-point range.

[17] In the NCAA tournament, Allen scored 29 points in Duke's 71–64 win over Yale, helping the team to reach the Sweet Sixteen.

[38] Duke reached the second round of the NCAA tournament, where the team was upset by South Carolina; Allen scored 20 points in the defeat.

[46] On March 9, 2018, Allen was assessed a flagrant-one foul in the ACC tournament for tripping North Carolina's Garrison Brooks with what was called a "hip check.

[52] On April 10, 2019, Allen scored a career-high 40 points with seven rebounds, four assists, a steal and a block in a 137–143 overtime loss to the Los Angeles Clippers.

[53] On July 6, 2019, Allen was traded, alongside Jae Crowder, Kyle Korver, the draft rights to Darius Bazley and a future first-round pick, to the Memphis Grizzlies in exchange for Mike Conley.

[54] On July 11, he was ejected from an NBA Summer League game between the Grizzlies and the Boston Celtics after committing back-to-back flagrant fouls that were described by commentators as "cheap shots".

[59] On August 5, 2020, Allen scored a season-high 20 points and logged a career-high six 3-pointers along with two rebounds and one assist in 26 minutes of action in a 115–124 loss to the Utah Jazz.

[62] On December 23, he made his season debut for the Grizzlies, logging six points, two rebounds and five assists in a 119–131 loss to the San Antonio Spurs.

[68] On August 7, 2021, Allen was traded by the Grizzlies to the Milwaukee Bucks for Sam Merrill and two future second-round draft picks.

[69] Bucks general manager Jon Horst commented, "Grayson is a talented guard... His three-point shooting, energy, toughness and basketball IQ make him a great addition, and we're excited to welcome him to Milwaukee.

"[79] On April 22, 2022, during Game 3 of the first round of the playoffs, Allen scored a then-playoff career-high 22 points in a 111–81 win over the Chicago Bulls.

[82] On September 27, 2023, Allen, alongside Portland Trail Blazers players Jusuf Nurkić, Nassir Little and Keon Johnson, was traded to the Phoenix Suns as part of a three-team trade that sent Damian Lillard to the Milwaukee Bucks and Jrue Holiday, Deandre Ayton, Toumani Camara and a 2029 first-round draft pick to the Portland Trail Blazers.

[83] On November 6, Allen scored a season-high 26 points as a starter with a career-high 8-of-13 three-pointers made in a tense 116–115 overtime win over the Chicago Bulls.

[84] On Christmas Day, Allen scored a new season high with 32 points made as a starter while making 8/17 three-pointers in a loss to the Dallas Mavericks.

The three-pointers made that game tied a franchise record previously set multiple times between former Suns players Quentin Richardson, Channing Frye, Aron Baynes, Cameron Johnson, and Landry Shamet.