As Zipcar considers going public, Whitman is the third well-known executive in recent months to join its board.
Whitman led eBay from 1998 to 2008. During her tenure, the company grew from a 30-employee operation with $4 million in annual revenue to one with 15,000 employees and nearly $8 billion in revenue.
Whitman replaces Zipcar investor and board member Jim Gerson.
Earlier this month, Whitman re-joined Procter & Gamble's board of directors after a two-year absence. She first joined board in 2003 and stepped down in late 2008 before launching her bid for governor. Whitman, a Republican, also resigned from the boards of eBay and DreamWorks Animation SKG. Earlier she joined HP's board as well.
She lost to Jerry Brown after waging the costliest state-level campaign in U.S. history. Her spending included $144 million from her personal fortune, but Brown bucked national GOP momentum in November and won by 13 percentage points.
That same month, the company said that it received $20 million of its latest round of Series G financing from Meritech Capital Partners, along with $1 million
Zipcar's fund raising and board appointments come as the company considers going public.
Zipcar told the Securities and Exchange Commission in June that it plans to make an initial public offering to raise about $75 million. A date has not been set.