In 1882 Coleman returned from Germany and was appointed Professor of Geology and Natural History at Victoria University, Cobourg. When Victoria joined the University of Toronto in 1891, Coleman became a professor in the School of Practical Science. In 1901 he was appointed Head of the Department of Geology, a position he held until retirement in 1922. In 1919 Coleman became Dean of the Faculty of Arts, University of Toronto. 

Coleman worked during the summers for the Ontario Bureau of Mines from 1893 to 1909 and later from 1931 to 1934. He published forty reports and papers on Ontario building stone, gold, copper, iron, nickel, corundum, Precambrian geology and glacial geology after mapping and studying these in the field. One of the geological maps from his 1913 report The Nickel Industry with Special Reference to the Sudbury Region "…enjoyed a wider circulation than any other geological map ever printed. "¹. He played an important role in the evolving theory of the Sudbury ore genesis and classification.

Coleman was instrumental in helping establish the Museum of Geology, one of the five original museums that comprised the Royal Ontario Museum. He was appointed first Director of the Royal Ontario Museum of Geology (1914 – 1922).  Coleman’s rock specimens collected from his trips across Canada and around the world were central  to the Museum’s mineralogical collections.

Active in Geological and Scientific Societies in Canada, the United States, and Great Britain, Coleman was considered one of the world's leading earth scientists.  He held the presidency of many of these associations and was awarded their highest honours.