Andy Haste, former chief executive of More Than insurance group RSA, said the business needed to undergo "significant change" even though this would hit profits in the short term.
His appointment comes weeks after Wonga agreed with regulators that it would pay £2.6 million in compensation after chasing struggling customers with fake legal letters to pressurise them into paying up.
Mr Haste said the company, which has been criticised by MPs over interest rates of more than 5,000%, must review rates, fees and charges and no be seen as targeting "the young and the vulnerable".
He said: "This is a sector and Wonga is a company that needs to go through significant change if it is to have a sustainable future.
"Some serious mistakes have been made. The company admitted those mistakes and it has apologised for those mistakes.
"Wonga has understandably faced a lot of criticism and I know that we need to repair our reputation and regain our right to be an accepted part of the financial services sector."
The new chairman pointed to the rapid growth of the company, which has more than a million customers after being founded seven years ago to explain how "some of our systems, our processes, our controls, haven't kept pace".
Mr Haste said a review of its customer base and products would ensure it was "only lending to people who can reasonably afford our loans".