Lillian Moller married Frank Bunker Gilbreth, a building contractor and efficiency expert, in Oakland, California. Their partnership would become a cornerstone of their professional and personal lives.

Lillian and Frank had twelve children: Anne Moller Gilbreth Barney, Ernestine Gilbreth Carey, Frank Bunker Gilbreth Jr., Dan Gilbreth, John Gilbreth, Lillian Moller Gilbreth Johnson, Frederick Moller Gilbreth, Martha Gilbreth Tallman, William Gilbreth, Robert Moller Gilbreth, Jane Moller Gilbreth Heppes, and Mary Elizabeth Gilbreth. Raising this large family while simultaneously pursuing professional careers presented unique challenges and opportunities for applying efficiency principles.

Frank Jr. and Ernestine Gilbreth Carey publish 'Cheaper by the Dozen,' a humorous and affectionate memoir of their childhood. The book gained widespread popularity and introduced the Gilbreth family to a broader audience. This helped popularize time-and-motion studies.

Frank Jr. and Ernestine Gilbreth Carey published 'Belles on Their Toes,' a sequel to 'Cheaper by the Dozen,' focusing on Lillian's life after Frank's death and how she managed the family. These books further cemented the Gilbreth family's legacy.