Allison Johnson, HP's senior vice president of marketing, was one of Fiorina's closest advisers at the company, having led the company's public-relations campaign during the contentious proxy battle over the of Compaq Computer as well as the " " advertising campaign that followed the deal's closure.
A source close to the company said Johnson's decision was unrelated to Fiorina's departure and made before the board's request for Fiorina's resignation.
Another source said HP's board and executives asked Johnson to stay on through last week's CEO announcement and this week's quarterly earnings announcement.
Johnson was not immediately available for comment.
The marketing guru is leaving HP to take a top marketing post at Apple Computer. She will become vice president of worldwide marketing communications, reporting to CEO Steve Jobs, an Apple representative told News.com.
At Apple, Johnson will be responsible for the company's global advertising and related efforts. She joins as Apple's image is soaring. The Mac and iPod maker earned the top spot in a recent survey regarding corporate brands.
Johnson had previously worked at Apple and also has held posts at IBM and Netscape Communications before joining HP in 1999.
At HP, Johnson's role stretched beyond marketing under Fiorina. She was described by one former co-worker as the "prime minister" of HP, BusinessWeek reporter Peter Burrows"Backfire: Carly Fiorina's High-Stakes Battle for the Soul of Hewlett-Packard," which traced the HP-Compaq fight.
In an e-mail to staff Monday afternoon, Chief Marketing Officer Mike Winkler said that for the time being, the leadership of HP's corporate marketing team will report to him. He also reiterated that Johnson's move was planned before Fiorina's resignation.
"This was a personal decision made prior to and independent of the change in CEO," Winkler said in the e-mail, which was seen by News.com. Winkler added that Johnson's departure "does not imply any changes to the strategy and direction of the marketing function or the company overall."
Word of Johnson's planned departure comes less than a week after Fiorina was.
At HP, Johnson served as vice president of global brand and communications, before beingas HP launched its plans to buy Compaq.
Despite being rivals in the consumer PC market, HP and Apple have been collaborating lately. Since last year, HP has beenand also distributing Apple's iTunes music software on HP laptops and desktops.
CNET News.com's Richard Shim contributed to this report.