[4] On June 23, 2017, Sumner was selected 52nd overall in the 2017 NBA draft by the New Orleans Pelicans, only to be traded to the Indiana Pacers in exchange for cash considerations.

In the 2017–18 season, Sumner struggled for minutes and spent significant time with the Pacers' G-League team, the Fort Wayne Mad Ants.

Sumner's limited minutes continued into the beginning of the 2018–19 season, but when teammate Victor Oladipo suffered a season-ending injury of a quadriceps tendon rupture his playing time increased significantly, and was given his first NBA start, against the Golden State Warriors.

[11] On October 6, 2021, Sumner was traded, alongside a 2025 second-round pick, to the Brooklyn Nets in exchange for the draft rights to Juan Pablo Vaulet.

[17] On November 29, 2023, Sumner signed with Žalgiris Kaunas of the Lithuanian Basketball League (LKL) and the EuroLeague.