Cecile Smith was educated in convent schools. In 1886 she went to Paris, where she studied painting with Alexandre Cabanel and Édouard Detaille. Within the next three years she married Josiah W. Wentworth, and it was as Mme C.E. Wentworth that she first exhibited at the Paris Salon in 1889. She continued to exhibit there for 30 years. Wentworth became widely noted as a portraitist, and over the years her sitters included President Theodore Roosevelt, President William Howard Taft, Queen Alexandra of England (a portrait commissioned by the king of Spain), and General John J. Pershing. Her portrait of Pope Leo XIII won a bronze medal at the Universal Exposition in Paris in 1900 and prompted the pope to decorate her with the title of grand commander of the Order of the Holy Sepulchre and to make her a papal marchesa. (The portrait later hung in the Vatican Museum.)

Wentworth was made a chevalier of the Legion of Honour in 1901. She was one of the very few woman artists to have her works acquired by the Luxembourg Museum. Works by Wentworth were also acquired by the Metropolitan Museum of Art in New York City, the Corcoran Gallery in Washington, D.C., and other leading museums.