The former Glasgow hooker made way for Peter Horne at the start of the year but returns to the role following Horne's promotion to the Warriors coaching staff.
MacArthur will be assisted by Frazier Climo while Elias Caven has become player/coach and will lead strength and conditioning training.
MacArthur led the Bulls to inaugural FOSROC Super6 success. He came through the Ayr youth system before playing professional rugby with Glasgow Warriors and Scotland.
Frazier Climo enjoyed a succesfull playing career at Millbrae. He was initially involved in the Bulls coaching staff in 2019, working under Peter Murchie as skills coach. Since then he has occupied a player/coach role with the 1st XV.