The online retailer said Dan Finlay would take over immediately after spending nearly three years at Debenhams, which Boohoo owns.
Mr Ashley’s Frasers Group had been pushing for Boohoo to install its founder in recent weeks after John Little announced he was stepping down from the online clothes retailer in October.
On October 24, Frasers – which is Boohoo’s largest shareholder with a 27% stake – called a meeting of Boohoo investors to back its plans to appoint Mr Ashley.
In an open letter, Frasers said it was “in the best interests of Boohoo, its shareholders and stakeholders” to appoint Mr Ashley “without delay”.
Frasers said Boohoo had been the victim of “long-term mismanagement” and “destruction of value”.
But on Friday, Boohoo deputy chairman Alistair McGeorge said the board was “unanimous” in its decision to appoint Mr Finley.
Mr McGeorge called the new chief executive “one of the outstanding leaders in a new generation of digital retailers”.
“Dan and his team have successfully transformed Debenhams from a failing department store, creating a new business model that is a capital-light, equity-light and high-growth market.”
It follows Mr Little’s announcement in mid-October that he would step down after five years as boss.
At the same time, Boohoo revealed plans for a strategic review, sparking speculation about a potential break-up of the business.
Boohoo also reported that revenue fell 15% to £620m for the six months to 31 August.
Mr Finlay said Boohoo had “brilliant brands and people” and that it would look to “extend” the business model pursued by Debenhams in recent years across the group.
Boohoo bought Debenhams out of administration in a £55m deal in 2021.