The Bay of Plenty MP takes the helm after receiving more votes than incumbent leader Simon Bridges at an emergency caucus meeting on Friday afternoon.
"There is no Team Todd, there is no Team Nikki, or anyone else - there is only Team National," Muller said in a statement sent to media on Friday afternoon.
"New Zealanders need a National Government with the experience and management skills to get our country through the worst crisis since the end of the Second World War.
"My focus as leader is our country's economic recovery and the strengthening of every community throughout New Zealand."
It is not yet known what proportion of National's 56 caucus members voted for Muller.
The result of the vote, which was organised earlier this week after Muller confirmed he would contest the party leadership, means Bridges' two-year stint as Opposition leader is now over.
It caps off a tumultuous few months for Bridges, who has guided National to a series of horror poll results and seen his own popularity plummet as Prime Minister Jacinda Ardern's skyrockets.
Following months of unrest within the party, Muller made his move this week after a devastating Newshub-Reid Research poll showed Labour would, as it stands, run away with the 2020 general election.
That poll - which proved to be the final nail in the coffin for the embattled Bridges - showed National had just 30.6 percent support from New Zealanders, while Labour had jumped to nearly double that at 56.6 percent. Less than one in 20 wanted Bridges as Prime Minister.
That came hot on the heels of an embarrassing gaffe from Bridges on his own Facebook page, in which an attack on Ardern's handling of the COVID-19 crisis backfired, causing many National supporters to announce they'd be switching sides.
Bridges had never really looked like providing the steady leadership his predecessors Sir John Key and Bill English brought to the role.
He's been embroiled in a number of scandals that have rankled Kiwis and damaged the party's reputation; from the Jami-Lee Ross saga, to the $113,000 in taxpayer money he spent on a roadshow around New Zealand, to his rumoured meeting with the head of communist China's 'secret police'.