The appointment of retired general Constantino Chiwenga at the senior ZANU-PF position was announced in a statement by the presidential press secretary.
Kembo Mohadi, a long-serving state minister, was also named ZANU-PF vice president.
Chiwenga, who resigned this week from his military role, was instrumental in a coup last month that eventually forced 93-year-old Mugabe to hand in his resignation letter, after ruling the country for 37 years.
The appointments are seen as a first step towards their elevation to vice president of the country.
Ruling party deputies are traditionally named state vice presidents, although this is not legally required. The president can have two deputies.
Chiwenga is one of several high-ranking military officials who have been given important roles in Zimbabwe after Mugabe’s ousting.
Earlier this month, two ministerial posts were given to senior military officials and war veterans who played a role in the brief coup.
Mnangagwa, like Mugabe, is a veteran of the struggle for independence from Britain. He was considered by the military to be an appropriate replacement for the former president, who, at 93, was Africa’s oldest leader.
Since being sworn in, Mnangagwa has been under pressure from opposition parties and the public to revive Zimbabwe’s economy and implement political reforms in an attempt to fight corruption.
On Thursday, Mnangagwa vowed to build a “new Zimbabwe” based on honesty, transparency and accountability, in an attempt to attract foreign investors.
Zimbabwe is scheduled to hold presidential elections in 2018, in which Mnangagwa will run as a candidate for ZANU-PF.