In Chicago he did a few designs for stained glass and a great deal of advertising and poster work for the Marshall Field and Co. department store. His career as an illustrator began in Chicago with cover work for magazines. In late 1909 or early 1910 MacLellan and Edward Wilson, a friend of MacLellan since boyhood, recalling the inspiration they had received from Howard Pyle’s lectures at the Art Institute of Chicago in 1905, decided to move to Wilmington to attend Pyle’s weekly lectures and obtain his criticisms. MacLellan occupied one of the Pyle studios at 1305 N. Franklin Street from 1910 until his death.

In 1926 he received special permission to enter the professional class at the Pennsylvania Academy of the Fine Arts, where he studied still life and life painting, apparently in reparation for taking up easel painting. About 1930 MacLellan began to have trouble with his eyes and gave up illustration to concentrate on portraiture and figure studies. He also became an expert on early American furniture, organizing an exhibition of it for the Wilmington Society of the Fine Arts in 1926 and serving an an advisor and agent for Henry Francis duPont in the formation of the Winterthur Museum collections as well as in the formation of other private collections. MacLellan was also one of the Studio Group teachers from 1943 to 1958.

MacLellan specialized in magazine covers. His work until about 1920 was largely of a broadly humorous nature — naughty children or domestic scenes   done in a vignette style. After 1920 his covers tended to be “pretty girl” subjects.