LONDON — GlaxoSmithKline promoted Emma Walmsley to succeed Andrew Witty as chief executive officer when he retires, singling out Britain’s largest drugmaker as the only major global pharmaceutical company led by a woman.

Walmsley currently leads Glaxo’s consumer health business. She will join the board on Jan. 1 and take the helm of the London-based company on March 31, Glaxo said in a statement on Tuesday.

The 47-year-old Oxford graduate, whose background is in marketing rather than science, will be tasked with piloting cutting edge treatments for cancer and infectious diseases through clinical tests and onto pharmacy shelves to boost earnings and revive Glaxo’s shares. Chairman Phil Hampton had said he was looking outside as well as within the company for a new leader.

‘‘I think it’s actually better to have somebody who’s more familiar with Glaxo, from working on the inside, rather than an external candidate,’’ said Stephen Bailey, a fund manager at Liontrust Investment Partners LLP in London with 800 million pounds ($1 billion) under management, including Glaxo shares.

Under Walmsley’s leadership, core operating margins at the consumer unit doubled to 14 percent in the quarter ended June from a year earlier, bringing Glaxo closer to its aim of 20 percent operating profit margin for the division by 2020.

Picking Walmsley means Glaxo probably will retain its consumer business rather than split it off, analysts at Liberum Capital said in a note to clients.