At international level Harder has previously taken up assistant coaching positions with the Norwegian Football Federation with both the U23 Women and U21 Men's teams respectively.
[5] He was an assistant coach in the American collegiate soccer system for the NJIT Highlanders and the Yale Bulldogs.
[10] In December 2020, Harder was appointed head coach of West Ham United Women, on a two-and-a-half-year contract.
[4] He led West Ham to sixth place in the 2021–22 FA WSL, with their highest ever points total in a season.
[17][18] After winning both the 2022 Toppserien and the Norwegian Women's Cup, Harder mutually terminated his employment in May 2023; SK Brann were fourth in the league at the time.
[5][11] He has a degree in sports science from Unitec Institute of Technology and studied for one semester at Virginia Tech.