Moffett grew up on a farm. He began his art training at the Cummins School of Art in Des Moines. By 1908 he was in Chicago where he enrolled at the Academy of Fine Arts. After a year homesteading in South Dakota (1910) he resumed studies at the Art Institute under Walcott and Vanderpool. The summer of 1913 was spent painting under Hawthorne in Provincetown followed by a year in New York at the Art Students League. Another summer brought him back to Provincetown the following summer where he finally settled. He took a studio at Day's Lumber Yard next door to Edwin Dickinson. The two became close friends and they often shared models. Dickinson later wrote the forward to Moffett's autobiography.
Moffett served in World War I, and then married Dorothy Lake Gregory, als an artist, in 1920. In 1928 he began showing at the Frank Rehn Gallery and in 1930 he served on the jury of the Carnegie International.
In addition to painting the scene at Provincetown, he was also a muralist. His mural works include: Captain Alezur Holyoke's Exploring Party on the Connecticut River for the Holyoke, Massachusetts Post Office (1936); A Skirmish between British and Colonists for the Somerville, Massachusetts Post Officer (1939); and The First Stone and Tavern for the Revere, Massachusetts Post Office (1939). The Abbey Mural Fund of the NAD awarded him and Louis Bouch‚ the commission for the murals for the Eisenhower Museum, Abilene, Kansas (1954-56).
About 1960 Moffett became interested in archeology and became active as a conservationist working to establish public parklands in Provincetown. He was active in the founding of the Provincetown Art Association and published a history of it Art in Narrow Streets, the First 33 Years of the Provincetown Art Association, in 1964.
Moffett was nomination to the NAD by George Elmer Brown.