Utah Jazz (2006–2013)
Millsap was selected by the Utah Jazz with the 47th overall pick in the 2006 NBA draft, and later signed his rookie scale contract with the Jazz on August 2, 2006. In late 2006, some sports journalists were referring to Millsap as a potential Rookie of the Year candidate, an award not traditionally given to players taken so late in the draft. Though the award ultimately went to Brandon Roy, Millsap had a strong first season, leading all rookies with six double-doubles. He finished the 2006–07 season with averages of around 7 points per game, 5 rebounds per game, nearly one steal and one block; he had season-highs of 20 points, 17 rebounds, 4 blocks, 4 steals, 3 assists, and 38 minutes. He played in all 82 of Utah's games. Early in the 2007–08 season, Millsap achieved a new career-high of 24 points during a win over the Cleveland Cavaliers. He later extended his career-high to 28 points against the Orlando Magic in late December 2007. His next career-high came in December 2008 against the Boston Celtics when he scored 32 points. Millsap had played in 194 straight games since being drafted by the Jazz in 2006; he missed his first game of his professional career on December 26, 2008 against the Dallas Mavericks due to a sprained posterior cruciate ligament injury in his knee suffered against the Milwaukee Bucks three days prior.
During the 2008–09 season, Millsap became the replacement for Carlos Boozer after the all-star forward suffered multiple injuries. Millsap's numbers subsequently soared, as he averaged 15.9 points and 10.3 rebounds over 38 starts mid-season.
On June 25, 2009, the Jazz tendered a qualifying offer to Millsap, making him a restricted free agent. A few weeks later, on July 10, 2009, he signed an offer sheet from the Portland Trail Blazers for $32 million over four years, including $6.2 million in the 2009–10 season, with a $5.6 million signing bonus, with $10.3 million to be paid within seven days of the contract's approval by the NBA. The Jazz had the right to match the offer, and did so seven days later, on July 17, 2009.
Millsap became Utah's permanent starting power forward in 2010–11 after Boozer joined the Chicago Bulls. Millsap started alongside newly acquired center Al Jefferson. On November 9, 2010, Millsap scored a career-high 46 points in a 116–114 overtime win over the Miami Heat. Millsap scored 11 points in 28 seconds at the end of regulation, including three three-pointers, one more than he had made in his entire career before then. He also hit a buzzer-beater to force overtime.
Atlanta Hawks (2013–2017)
On July 10, 2013, after spending the first seven years of his career with the Jazz, Millsap signed a two-year deal with the Atlanta Hawks. On January 30, 2014, Millsap was voted by the coaches to be a reserve on the 2014 East All-Star team. On March 18, 2014, Millsap recorded his first career triple-double with 19 points, 13 rebounds and 10 assists in a 118–113 overtime win over the Toronto Raptors.
On January 29, 2015, Millsap earned his second consecutive All-Star nod as a reserve for the Eastern Conference in the 2015 NBA All-Star Game. The Hawks sent four players to the All-Star game, all of whom were selected as reserves. He went on to help the Hawks finish with a 60–22 record in 2014–15, the team's best finish since 1993–94. They made it through to the Eastern Conference Finals where they were swept 4–0 by the Cleveland Cavaliers.
On July 9, 2015, Millsap re-signed with the Hawks to a three-year, $59 million contract. On January 16, 2016, he scored 21 points against the Brooklyn Nets to move past 10,000 for his career. On January 28, he earned his third consecutive All-Star nod as a reserve for the Eastern Conference in the 2016 NBA All-Star Game. On April 9, 2016, he recorded a season-high 31 points and 16 rebounds in a 118–107 win over the Boston Celtics. The Hawks finished the regular season as the fourth seed in the Eastern Conference with a 48–34 record. In the first round of the playoffs, the Hawks faced the fifth-seeded Boston Celtics, and in a Game 4 loss on April 24, Millsap scored 45 points – a career playoff high and one off his all-time best – and added 13 rebounds. The loss tied the series at 2–2. The Hawks went on to defeat the Celtics in six games and moved on to the semi-finals, where they were defeated in four games by the Cleveland Cavaliers.
In the Hawks' season opener on October 27, 2016, Millsap scored a game-high 28 points on 11-of-20 shooting in a 114–99 win over the Washington Wizards. On December 19, 2016, he scored a season-high 30 points, including the go-ahead 10-footer with 12.7 seconds left, to help the Hawks defeat the Oklahoma City Thunder 110–108. On January 1, 2017, he set a new season high with 32 points and had 13 rebounds in a 114–112 overtime win over the San Antonio Spurs, helping the Hawks snap a string of 11 straight losses to San Antonio since 2010. On January 26, he was named an Eastern Conference All-Star reserve for the 2017 NBA All-Star Game, earning his fourth straight All-Star nod. Three days later, he had a season-high 37 points, including the go-ahead layup with 27 seconds remaining in the fourth overtime, to lead the Hawks to a 142–139 win over the New York Knicks. He also had 19 rebounds, setting another season high, and seven assists.
Denver Nuggets (2017–present)
On July 13, 2017, Millsap signed a three-year, $90 million contract with the Denver Nuggets. In his debut for the Nuggets in their season opener on October 18, 2017, Millsap scored 19 points in a 106–96 loss to his former team, the Utah Jazz. On November 21, 2017, he was ruled out indefinitely after sustaining a left wrist injury. Five days later, he underwent successful reconstructive surgery on his left wrist, ruling him out for several months. He returned to action on February 27, 2018 against the Los Angeles Clippers, recording nine points and seven rebounds in 23½ minutes in a 122–120 loss. On March 30, 2018, he scored a season-high 36 points to help the Nuggets beat the Oklahoma City Thunder 126–125 in overtime.
On December 3, 2018, Millsap was named Western Conference Player of the Week for games played from Monday November 26 to Sunday December 2. It was his third career Player of the Week award and became the 18th Nugget in franchise history to win the award. Four days later, against the Charlotte Hornets, Millsap suffered a broken toe in his right foot. He subsequently missed eight games. On February 13, 2019, he matched a season high with 25 points in a 120–118 win over the Sacramento Kings. On March 14, he set a new season high with 33 points in a 100–99 win over the Dallas Mavericks.