She attended an art exhibition for Duncan Phillips at the Century Association in January, 1921. She met Duncan, and the two married in October of that year. Duncan was an art collector, and the couple expanded their collecting. Phillips moved to Washington, D.C. and into Duncan's Dupont Circle mansion. Duncan's mother also lived at the mansion. Phillips gave birth to her first child with Duncan, Mary Marjorie.
While visiting France in 1923, Phillips became interested in Impressionism. She found inspiration in the work of Pierre-Auguste Renoir and Paul Cézanne. She didn't only find inspiration in their work, but the couple began collecting their work. During that 1923 trip, they purchased Renoir's Luncheon of the Boating Party for $125,000. In 1924, their second and final child, Laughlin, is born.
In 1930, the couple had collected so much artwork that they had to leave the mansion, with the art, and move into a nearby house in the Foxhall neighborhood. The house was named "Dunmarlin". The couple had a summer home in Ebensburg, Pennsylvania. It was called Ormsby Lodge and Carriage House. Duncan died in 1966. In total, the couple had collected over 2,500 works of art, many which were on display at the museum. After his death, Phillips, who served as Associate Director of the Phillips Collection, became Director.