US Open champion McDowell is the first Northern Irishman to claim the award that dates back to 1951.
"What a treasure chest our members had to choose from this year but, in the end, I suspect that the abiding image of an exhausted Graeme McDowell clinching the Ryder Cup at Celtic Manor just tipped it in his favour," said Bill Elliott, chairman of the Association of Golf Writers.
"Beating Tiger (Woods in the Chevron World Challenge] in California a week ago didn't do the Ulsterman any harm either."
Colin Montgomerie's victorious Ryder Cup team finished second, just ahead of USPGA winner and European No 1 Kaymer, with Westwood having to settle for fourth place.
Votes were also cast for Laura Davies, a five-time winner on the Ladies European Tour, and Italian teenager Matteo Manassero for becoming the youngest winner on the European Tour, as well as Rory McIlroy, brothers Edoardo and Francesco Molinari, Ian Poulter, Justin Rose, Miguel Angel Jimenez and British and US Senior Open champion Bernhard Langer.
"I cannot thank the team around me, my family and management, enough for their support," said McDowell. "I will have the same attitude in 2011 as I did in 2010, to work hard and to try and win some tournaments."