Detroit Pistons (2016–2019) On June 23, 2016, Ellenson was selected by the
Detroit Pistons with the 18th overall pick in the
2016 NBA draft. On July 19, he signed his rookie scale contract with the Pistons. On October 26, he made his professional debut in a 109–91 loss to the
Toronto Raptors, recording two rebounds in two minutes off the bench. During his rookie season, Ellenson had multiple assignments with the
Grand Rapids Drive, the Pistons'
D-League affiliate. On February 9, 2019, Ellenson was released by the Pistons.
New York Knicks (2019) On February 20, 2019, Ellenson signed a 10-day contract with the
New York Knicks. After several productive games, he signed a standard contract with the Knicks on March 2. On June 30, 2019, Ellenson joined the Knicks for the 2019
NBA Summer League.
Brooklyn Nets (2019–2020) On July 17, 2019, Ellenson signed a
two-way contract with the
Brooklyn Nets. On January 3, 2020, the Nets waived Ellenson after he appeared in five games.
Raptors 905 (2020–2021) On January 21, 2020, Ellenson was acquired by the
Raptors 905 in a
sign-and-trade deal with the
Long Island Nets. On November 26, 2020, Ellenson signed a deal with the
Toronto Raptors. On December 19, he was waived by the team. On January 27, 2021, Ellenson was included in roster of Raptors 905.
Toronto Raptors (2021) On March 10, 2021, the
Toronto Raptors announced that they had signed Ellenson to a 10-day contract.
Monbus Obradoiro (2021–2022) On July 13, 2021, Ellenson signed with
Monbus Obradoiro of the
Liga ACB.
Joventut Badalona (2022–2023) On July 7, 2022, Ellenson signed with
Joventut Badalona of the
Liga ACB. On July 4, 2023, Ellenson signed with
Ibaraki Robots of the
B.League. On September 19, he was placed on the injured list. On January 9, 2024, his contract was terminated, he didn't play any game.
Wisconsin Herd (2024–2025) On October 15, 2024, Ellenson signed with the
Milwaukee Bucks, but was waived two days later. ==Career statistics==