Whitmore, who is quitting Experience Communications, the planning consultancy he co-founded in 2004, replaces Helen Tridgell, who quit Postar in August after five years in the role.
Tridgell's departure followed delays in bringing trackside and Tube sites into the Postar system. However, in July Postar announced that it was finally able to measure the London Underground audience in the same way as roadside audiences.
Whitmore will join Postar in mid-November. Ken New will continue in his role as Postar's chairman.
Whitmore said his departure from Experience was amicable. Greg Turzynski and David Reid, the other partners in the business, will continue to lead the company.
"I didn't enjoy it (working at Experience). I'm not an entrepreneur but you don't know that when you start a business. I found the work interesting and have an excellent relationship with David and Greg, but if you're not happy it's time to move on," he said.
New said Whitmore's agency background (he was the managing director of Mediaedge:cia before joining Experience) would be valuable. "He's a bright guy and I think he will take Postar forward with fresh eyes," he said.
Whitmore said his main goal was to make the Postar system more comprehensive.