Mildred Manning was the daughter of Ernest Le Souef, planner and director of the Perth Zoo, and his wife Ellie. One of four children who all grew up with the animals and birds of the zoo as playmates, Mildred Le Souef completed her education at Perth College and the University of Western Australia.

After graduating with a science degree in 1923, Mildred Le Souef taught biology part-time at Wesley College and Presbyterian Ladies' College. In 1930 she became a full-time teacher at Wesley College, a position she held until 1970, and then helped the new science master until 1976.

During World War II she was at first a receiver of messages, and then a spotter with the Volunteer Air Observer Corps at Crawley. In 1951 Mildred Le Souef married Bernard Manning (deceased 1961) - a Gilbert and Sullivan actor who also founded the Gilbert and Sullivan Society in Perth in 1951. Mildred Manning was a member of the Young Womens’ Christian Association (YWCA) and the Science Teachers' Association of Western Australia, who made her a Life Member in 1970.