Cecilia Beaux was born in Philadelphia in 1855. Her mother died just days after her birth, and Beaux and her sister went to live with their grandmother and aunts. Her adoptive family exposed her to fine art throughout her childhood and, once in school, Beaux excelled in her drawing classes and began training in the studio of Catherine A. Drinker, an artist and a cousin of her uncle Will Biddle. From 1881-1883 she attended life classes directed by William Sartain, who traveled to Philadelphia from New York to give criticisms. She also counted the Pennsylvania Academy of the Fine Arts master Thomas Eakins among her early influences, though she did not receive direct instruction from him.
Her first major success in painting was a double-portrait of her sister and nephew entitled Les Derniers Jours d'Enfance, exhibited first at the American Art Association, and in 1885 at the Pennsylvania Academy, where it won the Mary Smith Prize, the first of many prizes Beaux received during her lifetime. In 1887, the painting was exhibited at the Paris salon to critical acclaim. Beaux's reputation as a Philadelphia portraitist grew steadily with the execution of several portraits her in Chestnut Street studio, and in 1888 she traveled to Europe to continue her studio education.
In Paris, she joined the Academie Julien, where she received criticisms from Tony Robert Fleury and William Adolph Bougereau. She spent the summer in Concarneau, Brittany, where Alexander Harrison and Charles Lazar critiqued her work, and returned to Paris, where she attended the Academie Colarossi under and sought out private criticisms in the atelier of Benjamin Constant. She copied paintings and classical sculpture at the Louvre, and traveled throughout Europe to view the works of old masters. In England, she painted several portraits of her friends, the Darwins, before returning to Philadelphia in August of 1889. She traveled to Europe several more times in her life, including a trip in 1896 to see six of her paintings exhibited at the Salon de Champs de Mars. At the time this was an unprecedented number of paintings shown there by an American, and their strength earned her a membership in the Societé Nationale des Beaux-Arts.
In the 1890s, Beaux earned a living painting commissioned portraits at her Philadelphia studio, while experimenting with and refining her style and technique with portraits of friends and family such as Sita and Sarita, of her cousin Sarah Leavitt with her cat, The Dreamer, of her friend Caroline Smith, and Ernesta with Nurse, of her niece, who was a favorite sitter of Beaux's throughout her life. Beaux became the first full-time female faculty member at the Pennsylvania Academy of the Fine Arts in 1895, and continued teaching there until 1915.
In the late 1890s, Beaux painted several works for which she would be repeatedly honored, including Mother and Daughter, a double-portrait of Mrs. Clement A. Griscom and her daughter Frances, which won four gold medals at international exhibitions, and The Dancing Lesson, a double-portrait of Dorothea and Francesca Gilder, the daughters of Richard Watson Gilder, editor of Century Magazine and himself a devoted friend and supporter of Beaux. The Gilders, and especially Dorothea, were steady companions as well as sitters for Beaux throughout her adult life. In 1901 and 1902, Beaux painted Mrs. Theodore Roosevelt and her daughter Ethel in the White House, and in 1903, she was elected to the National Academy of Design.
By 1905 Beaux was living and working primarily in New York during the winter, and at "Green Alley," a home she built in Gloucester, Massachusetts, in the summer. She was introduced to Gloucester by her friend, the Harvard economist A. Piatt Andrew, and entertained a steady stream of intellectual, literary, and artistic friends such as Isabella Stuart Gardner, William James, and Thornton Oakley. Beaux continued to amass prizes and honors for her artwork, including an honorary doctorate at the University of Pennsylvania in 1908. She had solo exhibitions at Macbeth Gallery in 1910, the Corcoran Gallery in 1912, and M. Knoedler Gallery in 1915 and 1917. She had regular public speaking appearances, published articles, and interviews on such subjects as art education, women in art, and modernist art, the pervasive influence of which she eschewed as a passing fad.
In 1919, she traveled to war-torn Europe as the official portraitist of the United States War Portraits Commission painted the portraits of three European war heroes: Cardinal Mercier, Admiral Beatty, and Georges Clemenceau. In 1924, she broke her hip in Paris, and although she continued to paint, she would never again be the prolific painter of her earlier years due to the injury. She wrote her autobiography Background with Figures in 1930, and in 1935-1936, the American Academy of Arts and Letters held the largest exhibition of her work that was mounted during her lifetime. Beaux died in 1942 in Gloucester, at the age of 87.