John Sloan was born in the lumber town of Lock Haven, Pennsylvania in 1871 and lived there until his family moved to Philadelphia when he was five.  John attended Central High School with William Glackens, a fellow artist of the Eight.  John taught himself to etch and sketch and by 1891 at the age of 20, he was working as a commercial illustrator and obtained a job as a staff artist at the Philadelphia Inquirer newspaper.
 
Sloan attended the Pennsylvania Academy of the Fine Arts where he met Robert Henri and other artists of the Eight circle and they encouraged each other to work at oil painting. Sloan was a student of Thomas Anshutz and studied in his famous Antique Class from 1892 to 1893.

Sloan became known as the American Hogarth for his realist depiction of scenes of everyday people and life. In 1895, Sloan took a new position at the Philadelphia Press where he continued to work until 1903. By that time, original illustrations had been replaced by the work of photographers and Sloan continued his career as a painter and printmaker, no longer working to the same extent as an illustrator.

Sloan joined his friends in New York City where they (Robert Henri, George Luks, William Glackens, and Everett Shinn) all had worked in the movement known as new realism or modernism. Recognized as a talented narrative painter, John Sloan taught painting and exhibited his work in the new modern genre style depicting images of the city and its residents. Sloan married his wife, Dolly (born Anna M. Wall), in 1901.

By 1912, Sloan became art director of the Masses socialist magazine. In 1913-14, he exhibited two paintings and five etchings at the famed Armory Show and resumed full time teaching and painting as a faculty member at the Art Students' League in New York.

His students from the Art Students League included: 
Peggy Bacon
Alexander Calder
David Smith
Angna Enters
Helen Farr
Reginald Marsh
Aaron Bohrod
Lee Gatch
John Graham
Adolph Gottlieb
Barnett Newman

Sloan quit his job with the Masses and resigned from the Socialist Party. During this time, Sloan began his long association with Kraushaar Galleries and in 1918, he became president of the Society of Independent Artists.

From 1914 to 1919, Sloan devoted his time to plein aire painting and summered in the artist colony at Gloucester, MA. In 1919, Sloan and his wife, Dolly visited Santa Fe, NM and his subjects included nudes and the landscape. Sloan was elected to the National Academy and lived at 88 Washington Place, New York, NY in the 1920s.

He was elected President of the League in 1931 and retired from teaching in 1938. After the death of Sloan's first wife, Dolly, from coronary heart disease in 1943, he married Helen Farr, a former student. Helen Farr had assisted Sloan in his compilation of the information for his philosophical art publication based on his lectures entitled The Gist of Art . In 1944, Sloan married Helen Farr. In 1950, he was awarded the Gold Medal from the American Academy of Arts and Letters. John Sloan died of cancer in Hanover, NH in 1951.