Leon Max, who currently lives in California, exchanged contracts on the grade one listed stately home last week, 14 months after it was put up for sale by Lord Hesketh.
After ending 470 years of Hesketh family ownership of the house with the deal, the 52-year-old has now announced he will convert a fire-damaged wing into the base for his label, Max Studio, and use the property as an English home for him and his American wife, Ame Austin.
In an interview with The Sunday Times newspaper, he said: "It is a daunting task.
"Easton Neston had been on the market for a while. At first we thought it was maybe a little too much of a challenge, but it is an exceptional house and a liveable home. It is a great privilege to be able to live somewhere as beautiful as Easton Neston.
"After living in California for 30 years, I'm looking forward to a bit of rain."
Mr Max told the newspaper he planned to convert the Wren Wing of the house – which was badly damaged by a fire in 2002 – into the base for his expanding European business.
And a Georgian stable yard will become a call centre for the mail-order part of his company.
The upmarket fashion chain, founded by Mr Max in 1979, has clothing worn by Hollywood stars including Gwyneth Paltrow and Angelina Jolie. It has more than 80 stores in America and the Far East.
He plans to expand the company into Europe to compete with brands such as Karen Millen and Jigsaw.
The designer is also currently having a minimalistic concrete mansion built on cliffs in Malibu, California.
Easton Neston was first put up for sale by Lord Hesketh for £50 million in May last year, but after the 3,300-acre estate failed to sell quickly he split it into pieces, with Mr Max snapping up the main house and park for a much-reduced price of a little under £15 million.
Some of the original estate has yet to be sold, with a lodge house and 10 cottages in Hulcote village still up for grabs.