During the summer prior to his senior season, Cook announced that he would be transferring to Oak Hill Academy in Mouth of Wilson, Virginia.

[7] During Cook's senior season at Oak Hill Academy he led the Warriors to a 31–4 record,[8] averaging 19.1 points, 10.9 anssists, and 2.5 steals.

[11] Cook signed his letter of intent to play basketball at Duke University on November 4, 2010; his announcement was broadcast live on ESPNU.

Cook was a part of a five player recruiting class for Duke in 2011, which also included Austin Rivers, Alex Murphy, Marshall Plumlee and Michael Gbinije.

[17] Cook was a member of the 2015 NCAA national champion Duke team, averaging 15.3 points and 2.6 assists per game.

[22] On November 14, Cook made his professional debut in a 106–99 loss to the Maine Red Claws, recording 15 points, a rebound, an assist, and a steal in 36 minutes.

[24] On April 11, Cook was named Rookie of the Year after averaging 19.6 points and 5.4 assists to go along with 3.9 rebounds in 43 games while shooting 47 percent from the field and connecting on a team-high 86 three-pointers.

[29] On February 18, 2017, he was named the MVP of the 2017 D-League All-Star Game at the Mercedes-Benz Superdome after recording an 18-point, 12-assist double-double.

[31] He made his NBA debut the following day, recording two points, two rebounds, and two assists in 17 minutes off the bench, in a 96–89 victory over the Miami Heat.

[39] Cook was signed by the Atlanta Hawks for training camp and was waived on October 13, 2017, as one of the team's final preseason roster cuts.

[40] He signed a two-way contract with the Golden State Warriors on October 17, 2017, and was recalled to the team on November 13.

Cook got his first NBA start on December 6, against the Charlotte Hornets, and scored eight points with three assists and three rebounds in 22 minutes.

On March 16, 2018, Cook scored a then career-high 25 points with three assists, three steals, and four rebounds in a 93–98 loss to the Sacramento Kings.

[42] On March 29, Cook set another career-high 30 points in scoring along with three assists and four rebounds in a 116–107 loss to the Milwaukee Bucks.

As a result of this, the Warriors signed Cook to a two-year deal on April 8, 2018,[44] to help ensure his eligibility to play in the postseason.

[45] On April 14, Cook made his debut in NBA playoffs, coming off from bench with five points, two assists, four rebounds, and a block in a 113–92 victory over the San Antonio Spurs.

[56] On October 26, 2021, Cook signed with Lokomotiv Kuban of the VTB United League until the end of the 2021–22 season.