She grew up in Santa Monica, California, the daughter of Barbara May and Butch May, a volleyball player who had competed at a high level himself. She played indoor volleyball at Long Beach State, where she won an NCAA championship in 1998 and established herself as one of the premier setters in the college game before transitioning to beach. The move to beach volleyball required rebuilding her game around a two-person format in which there are no substitutions, no rotations, and no margin for a bad stretch of play, and she became the defensive anchor of a partnership with Walsh Jennings that won 112 consecutive matches across a stretch that covered an entire Olympic cycle. Her passing and defense were the foundation the partnership was built on: Walsh Jennings blocked and attacked at the net, and May-Treanor controlled everything behind her with a consistency that made the system function at a level no other pair in the sport could match

She married baseball player Matt Treanor in 2004. She retired after the London 2012 gold, having won three consecutive Olympic titles with Walsh Jennings and left the sport without a defeat at the Olympic level. Three Games, three golds, no losses. She retired after London 2012.