In his first season with the Lakers, and fourth in the Association, Cook appeared in 44 games, making one start and averaging 11.5 minutes per contest. Cook posted 5.1 points, 1.2 rebounds and 1.1 assists per game. He also drained 0.7 three-pointers per game, knocking down 36.5 percent of his three-point attempts. In an Oct. 27 win over Charlotte, Cook handed out a career-high-tying eight assists, while also pitching in 12 points and three rebounds. Cook had eight double-figure scoring games, headlined by a season-high 22-point, four-rebound performance in a Jan. 15 matchup against the Magic. In an Aug. 8 meeting with Indiana, Cook posted 21 points and matched his career-high with five three-pointers. Cook appeared in six postseason games as the Lakers defeated the Miami Heat in the NBA Finals from Orlando. In Game 2 of the Lakers' first-round series against Portland, Cook posted five points and three assists in eight points. In Game 1 of the Finals, Cook came off the bench and drilled his only three-point attempt. After 2019-20, Quinn Cook became a 2x NBA Champion.

In his third NBA season, Cook averaged 6.9 points, 2.1 rebounds and 1.6 assists while playing 14.3 minutes per game for Golden State. During his second campaign with the Warriors, Cook took a step back in playing time from the 22.4 minutes he was given in 2017-18. His per-36-minute averages, however, were significantly improved across most statistical categories. Cook shined when given more run, and he averaged 11.7 points, 4.2 assists and 4.0 rebounds in 28.4 minutes across 10 starts. Though Cook operated primarily as a reserve, he had some big moments that displayed his potential. He scored a season-high 27 points in a win over the Nets on Nov. 10 and drained at least three triples 11 times. Overall, Cook shot over 40 percent from deep for the third straight campaign. After appearing in just 47 regular-season games combined over his first two seasons, Cook's 74 games played in 2018-19 was highly encouraging. The Duke product went on to play in 17 postseason games for Golden State, averaging 11.4 minutes per contest. He posted per-game averages of 4.2 points and 1.1 rebounds in the playoffs while shooting 32.4 percent from deep. Cook scored a playoff-high 11 points while going 3-for-3 from beyond the arc against the Clippers in Game 2 of the first round.

After bouncing around between clubs for two seasons, Cook was given a more steady role with the Golden State Warriors in 2017-18. He spent much of the season with Santa Cruz in the G League, where he played 29 games and averaged 25.4 points along with 8.1 assists and 4.6 rebounds per contest. Cook also averaged 2.9 three-pointers per game in the G League and shot 43.7 percent from deep. When he was finally given a chance at the NBA level, Cook held his own. He played in 33 games for the Warriors, averaging 22.4 minutes per contest. The 6-foot-2 guard posted per-game averages of 9.5 points, 2.5 rebounds and 2.7 assists with Golden State. He also made 1.4 three-pointers per game and shot a very efficient 44.2 percent from deep. Cook impressed the Warriors enough to earn a two-year contract from the team. He scored a career-high 30 points while making all five of his three-point attempts against Milwaukee on March 29. Cook got his first taste of NBA postseason action with the Warriors, playing in 17 games during the team's run to the NBA Championship. In those contests, he posted per-game averages of 4.8 points and 1.4 rebounds.

Cook was not selected in the 2015 NBA Draft. Although he played six preseason games with the Cleveland Cavaliers prior to the 2015-16 campaign, he did not appear in any regular-season contests. In September of 2016 he signed a contract with the New Orleans Pelicans but was waived before the start of the regular season. Cook then latched on with the Canton Charge of the D-League, with whom he played 38 games. Cook impressed with Canton, posting per-game averages of 26.0 points, 6.7 assists and 2.5 three-pointers. He was also named MVP of the D-League All-Star Game after registering 18 points and 12 assists in the contest. His performance with Canton caught the eye of the Dallas Mavericks, who signed Cook to a 10-day contract in late February. In five games with Dallas, he totaled 27 points and 12 assists. However, he returned to the D-League on March 8. Soon thereafter, Cook signed on with the New Orleans Pelicans. In his seventh game with the Pelicans, he scored a career-high 22 points. Overall, Cook averaged 5.8 points and 1.6 assists with New Orleans.