Cowan was a key figure in the creation of the King Edward Memorial Hospital for Women, and became a member of its advisory board when it opened in 1916. She was made a justice of the Children's Court in 1915 and a justice of the peace in 1920. In 1921, Cowan was elected to the Legislative Assembly of Western Australia as a member of the Nationalist Party, becoming Australia's first female parliamentarian. She was defeated after just a single term, but maintained a high profile during her tenure and managed to secure the passage of several of her private member's bills.