Fortune has been with United as a coach since 2012, working on his badges, and he eventually took the under-23 assistant coach job in 2019.
United's head of academy, Nick Cox, said: "Everyone at the academy wishes Quinton the best of luck in his new role.
"All of our coaches, as well as the players, receive a huge amount of development and this is another example of the type of progress that an individual can make within the academy system.
"The U23s' promotion was a great achievement for Neil Wood and his staff so we'd like to thank Quinton for his role in that.
"We are fortunate to have a pool of excellent coaches in the professional development phase who are ready to take on increased responsibilities. They will work together to support the squad as we look to continue to build on the eight first-team debuts that were made last season."