Cecilia Beaux was born in Philadelphia, Pennsylvania, a daughter of Jean-Adolphe Beaux, a French silk manufacturer and his wife Cecilia Kent Leavitt. After Cecilia's mother died in childbirth, her father returned to France, leaving Cecilia and her older sister Aimée to be raised by their maternal relatives in Philadelphia and New York City. She was educated at home and spent two years at a Philadelphia finishing school. At age 16, she began art lessons with her cousin Catharine Ann Drinker Janvier, a well-known artist and writer. By 18, Cecilia was giving private art lessons herself and was appointed a drawing teacher at Miss Sanford's School. She did made lithographs, painted on porcelain, and did copy work for printer Thomas Sinclair, publishing her first work in St. Nicholas Magazine in 1873. In 1876, she began attending the Pennsylvania Academy of the Fine Arts, where she won the Mary Smith Prize at the exhibitions in 1885, 1887, 1891, and 1892.
In 1883, she opened her own studio in Philadelphia. During 1888–1889, she traveled and studied in Europe, taking lessons at the Académie Julian in Paris and from several leading artists. Back home in the USA, she became a highly sought-after portrait painter and her pictures were widely exhibited. In 1895 she was named the first woman instructor at her alma mater, the Pennsylvania Academy of the Fine Arts, and the following year she exhibited six portraits at the Paris Salon. She was elected to membership in the Société Nationale des Beaux-Arts and later to the National Academy of Design, and the American Academy of Arts and Letters. She moved to New York in 1898 and also built a summer house in Gloucester, Massachusetts. In 1930, she published an autobiography entitled Background with Figures.