Merki grew up in Portland, Oregon. At a young age, she contracted poliomyelitis. With discovery of the polio vaccine years away, she took up swimming as exercise to reduce the effects of the disease at the age of 8. Trained by long-time swim coach Jack Cody, Merki excelled at the sport, and at the age of 13 entered the swimming scene at the Amateur Athletic Union (AAU) national championships in 1939, setting American records in the 200, 400, and 800-freestyle events. Merki, who along with Multnomah Athletic Club teammates Brenda Helser, Suzanne Zimmerman, Geneva Klaus, Joyce Macrae, and Mary Anne Hansen, were known as "Cody's Kids" after their coach, figured to be a force at the 1940 Summer Olympics until the games were canceled by the events of World War II.