Denver Nuggets (2018–present)
On June 21, 2018, Porter was selected with the 14th overall pick by the Denver Nuggets. He fell from a possible first overall pick to the 14th pick with the Nuggets due to reports regarding his health. The Nuggets also discussed the possibility of having him sit out for his entire first season in the NBA in relation to his back problems. On July 3, 2018, Porter signed a multi-year contract with the Nuggets. On July 19, 2018, the Nuggets announced that Porter had undergone a second back surgery.
On October 31, 2019, Porter made his debut in NBA, coming off the bench with fifteen points, four rebounds and an assist in a 107–122 loss to the New Orleans Pelicans. On December 29, Porter made his first career start in the NBA, finishing with 19 points, along with six rebounds and an assist in 26 minutes in a 120–115 win over the Sacramento Kings. He set a new career high with 25 points in 23 minutes just four days later in a win against the Indiana Pacers. On August 4, 2020, returning from the suspension of the season due to the COVID-19 pandemic, Porter Jr. led the Nuggets to their first victory in the Orlando bubble, scoring a career-high 37 points in route to a 121–113 overtime win over Oklahoma City Thunder.
On March 24, 2017, Porter committed to play for Missouri, joining his father and his younger brother, Jontay Porter (who reclassified from the class of 2018).
Porter was injured in the first half of the season opener against Iowa State. Porter was expected to miss the remainder of the 2017–18 season because of a lower back injury which required surgery. On November 22, 2017, Porter had a successful microdiscectomy of his L3-L4 spinal discs. On February 22, 2018, Porter Jr. was cleared to practice with Missouri again, with the potential to return to play before the end of the season. NBA executives were encouraging him to play if he was feeling healthy enough to do so.
Porter would officially return to action on March 8, 2018 in the quarterfinals of the SEC Tournament. Porter would also play in the first round of the 2018 NCAA Division I Men's Basketball Tournament, where the team lost to Florida State. In both games, however, Porter came off of the bench to play instead of starting as he had on opening night, mainly as a precaution. On March 27, Porter announced his intention to forgo his final three seasons of collegiate eligibility and declared for the 2018 NBA draft.
In addition to younger brother Jontay, he has two older sisters named Bri and Cierra, along with 4 younger siblings.