Wanda Gag was the daughter of German immigrants engaged in the arts—her father was a painter, her mother a photographer. As a result, she was immersed in art during her childhood, which led her to attend art schools in St. Paul and Minneapolis. In 1917 she completed her training at the Art Students League in New York City.

During the 1920s Gag began making prints, eventually producing more than a hundred editions of lithographs, woodcuts, and other types of prints. In 1928 she published Millions of Cats, the first of a number of children's books written and illustrated by her. Throughout the 1930s and 1940s Gag worked as a printmaker, winning praise for her landscapes, still lifes, and interior subjects, which display her distinctive personal style. The American Institute of Graphic Arts honored Gag by including her work in its annual selections of fifty outstanding prints.