Dorothy Hamilton Brush was born on March 14, 1894 to Walter James Hamilton, a lawyer, and Mary Jane Adams. She had two sisters, Gladys and Margaret. They grew up in Ohio and Brush went to school at the Hathaway Brown School in Cleveland, Ohio. She attended Smith College where she was very active, serving as a delegate to the YWCA Silver Bay Camp in New York, as a house president, and as a Student Adviser. She also participated in a number of clubs and societies, including the Alpha Society, Il Tricolore, Blue Pencil, and the Debating Union. She wrote short stories and other works and served on the editing staff for a variety of college publications such as the Monthly Board, the Weekly Board, and the Class Book Board. During her junior year Brush was the class historian. She wrote the "History of Junior Year" for her class in the 1917 yearbook, as well as the words for the Ivy Song and for "Alma Mater." She graduated from Smith College in 1917.

In 1917 Brush married her first husband, Charles Francis Brush, Jr., son of Charles Brush, the creator of a new arc lamp and lighting system and founder of Brush Electric, which later merged with two other companies to become General Electric. While Charles Brush, Jr. was serving in the Ordinance Officers' Reserve Corp of the Army as a first Lieutenant from 1917 to 1919, Dorothy worked for various charities in Washington, D.C., and then in Sheffield, Alabama. In 1919 Charles and Dorothy embarked on a round-the-world trip that included a stop in Honolulu, Hawaii, and then returned to the United States to settle in the Cleveland, Ohio area where Charles established the Brush Research Laboratory. Their first child, Jane, was born in 1920 and their second child, Charles III, was born in 1923. Beginning in 1922 Brush volunteered for the Junior League in the Cleveland area. Her work in a prenatal clinic led to her increased awareness, and advocacy, of women's health issues and birth control. Along with several friends and supporters, Brush played a large part in the establishment of the Maternal Health Association in 1928, the precursor to Planned Parenthood of Cleveland established in 1966.

In 1927 Brush's daughter Jane became ill with pneumonia and needed a blood transfusion, for which her father Charles had volunteered to donate his blood. Jane did not recover and died, and Charles suffered complications from the transfusion, dying a week later. In an effort to honor the memory of his son, Charles Brush, Sr., established, at Dorothy's behest, the Brush Foundation for the purpose of funding research in population control. Dorothy Brush continued her work with the Maternal Health Association but also became increasingly involved with the Foundation, serving as an administrator and later as Chairman from 1957 to 1963. The Brush Foundation still exists and has broadened its mission to include research and education on a wide variety of birth control issues.

In 1929 Dorothy Brush married Alexander Colclough Dick and they moved to New York City. In 1930 their daughter Sylvia was born. That same year Dorothy began the work with Margaret Sanger that would define her activities for the rest of her life. She volunteered in Sanger's clinic and later in the 1930s traveled with her throughout Asia and Europe as a missionary for birth control and family planning. Brush placed great value on Sanger's work and, as a Smith College alumna, knew about the Sophia Smith Collection (SSC) and its collecting efforts in women's history. She wrote to Margaret Storrs Grierson, director of the SSC, in 1946 and together they convinced Sanger to donate her papers to Smith (portions of her professional papers were already given to the Library of Congress). Brush was also instrumental in Sanger's receiving an Honorary Doctor of Laws degree from Smith College in 1949 and led the effort to nominate her for a Nobel Prize.

Even as her work with Margaret Sanger and the birth control movement became a central focus, Brush continued her writing. She co-wrote (using her maiden name Dorothy Adams Hamilton) with Walter S. Hayward the books, (1943) and (1943). Brush also wrote travel articles for the magazine and a few children's plays for the Samuel French Company that were published in the late 1920s. In addition to these published materials, Brush also authored a number of manuscripts on women in Japan, Margaret Sanger, birth control, and on menopause that were never published.

In 1947 Brush and her husband Alexander Dick divorced. In 1952 Sanger and Brush saw the fruit of their work in the international birth control movement with the establishment of the International Planned Parenthood Federation. Also in 1952 Brush started the IPPF newsletter, , which received funding from the Brush Foundation, and she served as its editor until 1956. Although she had intended to retire at this point in part due to health problems, in 1957 Brush agreed to serve as Honorary Advisor for Field Work Services. Many field offices faced political and legal obstacles in continuing their work with family planning and birth control, and Brush and the rest of the field officers helped to support their efforts by locating funding, recruiting volunteers, and generally providing solid, reliable information. Brush continued this work for 4 years, finally retiring in 1961.

In 1962 Brush married Dr. Lewis C. Walmsley, an educational missionary and Professor at the University of Toronto whom she had first met in 1937 through Margaret Sanger. In the early 1960s Brush spent an increasing amount of time at Sanger's home in Arizona due to Sanger's failing health, assisting her until Sanger passed away in 1966. Brush suffered herself during these years and many years before from emphysema and other maladies, and her travel and activities were often restricted.

Because of her continued work with and dedication to Smith College, Brush was awarded a Smith College Medal in October 1967. The medal noted her tireless work with family planning and women's health, her unfailing support of Margaret Sanger, and her numerous contributions to the Sophia Smith Collection and Smith College. Dorothy Brush died a few months later on June 4, 1968.