Mortensen attended Long Island Lutheran Middle and High School during her senior year.
[1] Following her high school career, Mortensen played college basketball for Miami from 2016 to 2019.
[2][3] In 2023, Mortensen played for the Cockburn Cougars in the Australian NBL1 West,[2] where she averaged 24.7 points and 11.1 rebounds per game.
[6] Mortensen started the 2023–2024 season with Cab Estepona in the Liga Femenina Challenge where she averaged 5.0 points in 11 games.
[8] In May 2015, Sarah led Denmark U18 to the Nordic Championship, while being named the tournament MVP.