The son of a railroad worker, William Glackens began his career as an artist-reporter for the Philadelphia Record and Press in 1891. The following year he joined the Philadelphia Press, working with John Sloan, who had been a classmate at Philadelphia's Central High School, and with George Luks and Everett Shinn. He attended evening classes at the Pennsylvania Academy of the Fine Arts, studying with Thomas Anshutz and Henry Thouron. Through Sloan, Glackens met Robert Henri, who became the leader of the group known as the Eight. Henri and Glackens shared a studio in 1894, the year of Glackens's first exhibition, at the 64th Annual of the Pennsylvania Academy of the Fine Arts. The following year, the two traveled to Europe, studying the Dutch Old Masters in Holland and painting in Paris and Fontainebleau.
Glackens returned to the United States in 1896, settling in New York and renting a studio with Luks. Freelance commercial-illustration work for newspapers and magazines would sustain him throughout the next decade. In 1898 he covered the Spanish-American War for McClure's Magazine. In 1901, along Henri, Sloan, and others, Glackens participated in an exhibition at the Allen Gallery in New York, receiving favorable reviews in the New York Evening Sun. His marriage in 1904 to Edith Dimock, an art student and the daughter of a wealthy Hartford silk manufacturer, brought financial security.
Glackens was elected a member of the Society of American Artists in 1905. When that organization and the Academy merged in 1906, he automatically became an Associate of the Academy. The rejection of several submissions by Glackens and Luks for the 1907 Academy annual exhibition spurred Henri to form the group that subsequently became the Eight. The group's landmark exhibition of 1908, held at the Macbeth Galleries in New York, included works by Arthur B. Davies, Glackens, Henri, Ernest Lawson, Luks, Maurice Prendergast, Shinn, and Sloan.
Like Henri, Luks, and Sloan, Glackens produced contemporary views of the streets, parks, and restaurants of New York that were distinguished by a direct and somber objectivity; his during this period work reflected his association with Henri. In 1910 Glackens helped Henri organize the Exhibition of Independent Artists, and in 1912-13 he chaired the committee that selected American entries for the Armory Show of 1913. Nonetheless, his work was seen frequently in the Academy's annual and winter exhibitions through the winter of 1915. In 1917 Glackens was elected the first president of the newly formed Society of Independent Artists.
Around 1910, Glackens turned to a new style, based on the later works of the French Impressionists and especially Pierre-Auguste Renoir. Studio-posed figure groups, nudes, still lifes, and beach scenes increasingly appear as subjects, rendered with an intense, high-keyed palette. He spent the summers of 1911-14 in Bellport, Long Island, producing a series of beach scenes incorporating these new influences.
In 1912 Glackens went abroad with $20,000 from his friend Dr. Albert C. Barnes to purchase an extraordinary group of Impressionist and Post-Impressionist paintings that would form the nucleus of the distinguished Barnes Foundation collection in Merion, Pennsylvania.
From 1925 to 1932 Glackens divided his time between his New York studio and France. In 1937 he was awarded the grand prize for painting at the Exposition Universelle in Paris. Following his sudden death the next year while visiting his friend the painter Charles Prendergast, the Whitney Museum of American Art in New York mounted a memorial exhibition of works selected by colleagues and fellow Academicians Guy Pène du Bois, Leon Kroll, and Eugene Speicher.