[1] Mortenson was born in Pierre, South Dakota and graduated from T. F. Riggs High School.
He is an enrolled member of the Cheyenne River Sioux Tribe.
He ran the 2010 reelection campaign of Public Utilities Commissioner Dusty Johnson, then worked in the Office of Governor Dennis Daugaard as a policy analyst, before attending the University of Virginia School of Law graduating with his J.D.
Mortenson first ran for the South Dakota House of Representatives from District 24 in 2020.
He won the primary in June 2020 and the general election on November 3, 2020.