She ran away from home at the age of 15 and did factory work before emigrating to the United States in 1930 to join her lover and fellow photographer Otto Hagel (1909–1973).

[2] During that time they began to photograph the brutal working conditions and suffering they saw around them, after acquiring a second-hand Leica camera.

In 1937 Mieth joined the staff of LIFE Magazine (only the second woman photographer to do so), and she and Otto (whom she married in 1940[2]), moved to New York.

He was then still a German citizen, so in order to escape internment during the Second World War the couple fled to a remote ranch near Santa Rosa in northern California.

Mieth continued to accept photography assignments for LIFE, while Hagel never left the Singing Hills Ranch.