Sonia Syngal served in a variety of executive roles before taking on the top job at Old Navy in 2016.

Gap Inc. said it promoted Sonia Syngal, the head of its Old Navy brand, to be its chief executive and appointed a board member to take over as board chairman from Bob Fisher, a son of the company's founders.

The apparel maker has struggled for years with sluggish sales as it lost ground to fast-fashion and off-price chains. In November, Art Peck quit as the retailer's CEO and Mr. Fisher temporarily stepped in. Earlier this year, Gap abandoned plans to break off Old Navy into a separate public company and started searching for a new leader.

Ms. Syngal, who will take over as CEO on March 23, joined the company in 2004. She served in a variety of executive roles before taking on the top job at Old Navy in 2016.

Ms. Syngal boosted the brand's store footprint in North America and Mexico and developed its e-commerce capabilities, Gap said Thursday.

An engineer by training, Ms. Syngal started her career as a product manager at Ford Motor Co. and a manufacturing executive at Sun Microsystems Inc., a maker of server computers.

Mr. Fisher will step aside as executive chairman of the company, effective March 23, and will be replaced by Bobby Martin, a former top executive of Walmart Inc.'s international business and a Gap board member since 2002. Mr. Fisher will remain on the Gap board.

Gap jettisoned its plan to hive off Old Navy in January, saying the split would have been too expensive and complex to pull off.

Old Navy had been a bright spot for its parent company, but the business has stalled more recently. Sales at Old Navy stores open at least a year fell for the first three quarters of the current fiscal year, after rising for most of 2018.

Gap also said Thursday that Elizabeth Smith, the former CEO of Bloomin' Brands Inc., and Amy Miles, the former CEO of Regal Entertainment Group, will join its board.

Shares of Gap, which have fallen more than 50% over the past year, fell 1.4% to $13.18 in after-hours trading.