Ellenson played basketball at Rice Lake High School, where he also competed in track and field. In August 2014, he was a member of the United States' National Team at the 2014 FIBA Under-17 World Championship in Dubai, scoring 21 points in the quarterfinal game with China. As a freshman in the 2011–12 season, Ellenson averaged 12.0 points, 7.0 rebounds, 1.5 assists and 1.5 blocks per game. As a sophomore in the 2012–13 season, Ellenson averaged 21.5 points, 11.8 rebounds, 3.5 assists and 1.8 blocks per game.
Career highlights and awards
Ellenson scored a career-high 48 points against Minnetonka High School on December 6, 2014
During his only season at Marquette, Ellenson averaged a near-double-double with 17 points and 9.7 rebounds per game, with 1.8 assists and 1.5 blocks in 33.5 minutes of action over 33 games. As a result, he earned All-Big East First Team, Big East All-Freshmen Team, and Big East Freshman of the Year honors. On April 5, 2016, Ellenson declared for the NBA draft, forgoing his final three years of college eligibility.
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.
On February 20, 2019, Ellenson signed a 10-day contract with the New York Knicks. After several productive games, Ellenson signed a standard contract with the Knicks on March 2. On June 30, 2019, Ellenson joined the Knicks for the 2019 NBA Summer League.
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.
The son of John and Holly Ellenson, he has two brothers and one sister. His father played two seasons of basketball at Marquette and two seasons at Wisconsin while his brother, Wally, was a member of the men's basketball and track & field teams there.
