The agency, which this week announced it was changing its name to Farm following its merger with the agency in June, won the business after a final pitch against JWT's fashion specialist, JWT Label.
Evans, which specialises in clothes for women sized 16 to 32, has hired the agency as it looks to increase its advertising presence in the UK.
Last year, the retailer spent £1 million on advertising, according to Nielsen Media Research, but it is understood that Arcadia is planning to raise its marketing spend for the brand this year.
Leith pitched for the brand before its merger with Farm. Its former managing director, Jeremy Pyne, and its chief executive, John Rowley, have both shifted into the newly merged agency's management line-up alongside Farm's chief, Rob Smith.
The review did not affect Carat, which handles the retailer's media account.
In June, Arcadia appointed Craik Jones Watson Mitchell Voelkel to handle direct marketing for the launch of its first branded credit-card. The card will work across the Arcadia businesses, including Top Shop, Miss Selfridge, Burton and Wallis.
Craik Jones, which won the account in a shoot-out against Personal, will create both off-line and online campaigns for the group, including in-store promotions and direct mail.
Arcadia and Leith both declined to comment.
Phillip Green, the British Monaco-based millionaire owns the Arcadia Group. He bought the business in 2002 and added it to his retail empire, which also includes BHS.