Although times were hard during the years of the Great Depression, and salaries were reduced frequently, Mrs. McMillen managed to sustain her firm of twenty-some employees, and by the end of World War II she had ensured that McMillen emerged as the most well-known residential decorating firm in America. In addition to designing and decorating the interiors of such wealthy Americans as Mr. and Mrs. Marshall Field, Millicent Rogers, Doris Duke, and Mabel Choate, McMillen undertook the design and decoration of the Steuben Showroom and the newly built Cosmopolitan Club, both in New York City.
McMillen's Miniature Rooms Exhibition, "Interiors of Tomorrow," toured the country between 1932 and 1935 to great acclaim, and a McMillen Gallery was opened in Houston in 1939. Of particular importance to McMillen from 1926 until her retirement in the early 1960s, was Grace Fakes, a teacher of Mrs. McMillen's at the New York School of Fine and Applied Arts. Miss Fakes was responsible for creating architectural interiors, furniture layout and design. She worked in tandem with Mrs. McMillen (by then Mrs. Archibald Brown) on the oversight of all projects.