 Yamauchi was born in Oxford, Oxfordshire, England to Dorothy and Norman Myers, and lived with her family until she was eight years old in Nairobi, Kenya.She was named after the Mara River which runs through Kenya where her parents lived for 25 years in total.Yamauchi started running with Oxford club Headington RoadRunners while still at school, but took up running seriously when she was an undergraduate at university, competing mainly in cross-country races.