Mainly remembered as a javelin thrower, she represented her country at the 1938 British Empire Games in Sydney, where she finished fourth in the women's javelin with a best throw of 118 feet 1/2 inch (35.98 m).In 1939, she won the inaugural women's javelin throw at the New Zealand athletics championships, with a winning distance of 111 feet 6 1/2 inches (34.00 m).
She won the national title again in 1940 and 1941, and her distance of 122 feet 6 inches (37.34 m) in 1940 remained the championship record until surpassed by Cleone Rivett-Carnac in 1952.
At the Auckland women's championships in 1941, Mitchell broke her own New Zealand record with a throw of 125 feet 8 inches (38.30 m).
She also at one time held the New Zealand women's long jump record, and was a competent high jumper, winning the Auckland championship in 1936.Mitchell was also known for her kauri carving, as an artist, a poet and a horsewoman.