SAN FRANCISCO • A top lieutenant in Silicon Valley is leaving the side of high-profile Internet executive Marissa Mayer to join another top honcho, Mr Jack Dorsey.

Ms Jacqueline Reses is departing Yahoo, where she was chief development officer, for Mr Dorsey's start-up Square, helping boost its executive team while dealing a blow to the Web portal, say people familiar with the matter.

Ms Reses' role changed earlier this year when she shifted her focus to Yahoo's spin-off of its stake in Alibaba Group that is worth about US$27 billion (S$37 billion), a source said in July.

Mr Dorsey was this month named permanent chief executive officer of Twitter, a role he also holds at Square ahead of its initial public offering. Adding her leadership could help lighten the load for Mr Dorsey amid concerns about his dual responsibilities.

Ms Mayer, who has been at the helm of Yahoo for more than three years, has struggled to build a stable management team as the company lost ground to Google, Facebook and others. Chief marketing officer Kathy Savitt, also hired under Ms Mayer's leadership, left to join movie studio STX Entertainment. Mr Kevin Gentzel, head of North America ad sales, and Mr Mike Kerns, senior vice-president, also left for other companies.

A Yahoo spokesman declined to comment. Square did not respond to a request for comment.

Ms Reses' exit comes at a delicate time, as Yahoo is planning to spin off its stake of about 15 per cent in Alibaba by the end of the year. Yahoo investors have assigned the majority of the company's stock value to its Asian assets with little left over for the Web portal's main business.

Square, a payments company known for its dongle that converts mobile phones into credit card readers, was started in 2009.

BLOOMBERG