McCormack replaces former deputy prime minister Barnaby Joyce, who was forced to resign last week amid public anger over an extramarital affair with his former press secretary with whom he is now expecting a child.
The leader of the National Party automatically becomes deputy prime minister under the terms of the coalition agreement with Prime Minister Malcolm Turnbull's Liberal party.
"I will honour the faith and trust and responsibility by always doing my best," McCormack told reporters in Canberra.
The appointment of McCormack will ease pressure on the government that was left reeling after revelations of Joyce's affair, a scandal that threatened to fracture the government.
Turnbull called Joyce's affair a "shocking error of judgment", to which Joyce responded by calling Turnbull "inept".
Although offering some respite to Turnbull, McCormack is little known around the country, weakening the government's re-election prospects.
