The appointment of Constantino Chiwenga was widely expected after his retirement earlier this month. He had to retire from the military to take up the position, according to the constitution.
Mnangagwa took power after Chiwenga led a military takeover in the southern African nation that forced Mugabe, then the world's oldest head of state at age 93, to resign amid impeachment proceedings after 37 years in charge.
Mnangagwa over the weekend appointed Chiwenga and Mohadi as vice presidents of the ruling ZANU-PF party.
Two former army generals already hold powerful posts in Mnangagwa's cabinet, while another general was appointed as ruling ZANU-PF party commissar at a congress on Nov 15.