Dean Clark was born in Royalton, Vermont in 1837 to Jedediah and Mary Clark. He helped his father on their farm and was still working there in 1860 at the age of 23. Dean married Harriet Horton Bardy in 1863 in Sudbury, Vermont where he worked as a physician. It is unclear what happened to Harriet. By 1870, Dean was traveling the country as a Spiritualist lecturer.

Dean lived in several communities, from Chicago, Illinois to Oakland and San Francisco, California, to Portland, Oregon from 1872 to 1886. He returned to the Boston area in 1887, but still travelled widely. He was speaker at the Liberal Spiritualist Society in Oakland at their 44th anniversary celebration in 1892. In 1900 he was a representative of the American National Spiritualist Association at the Paris Congress and visited London while abroad.

A Guide to Mediumship and Psychical Unfoldment by E. W. and M. H. Wallis contained several quotes from Dean dealing with mediumship, magnetism, and physical phenomena. One from the Banner of Light stated: “The word mediumship, as understood and used by Spiritualists, technically speaking, means a susceptibility to the influence, and more or less control, of decarnated spirits. Physiologically, it means a peculiar nervous susceptibility to what may be termed the ‘psychic force,’ which spirits use to move the mind or body, or both, of their moral instrument.” He added that, “Novices in mediumship have no business to assume obligations they are not fully qualified to fulfill.”

In the Progressive Thinker, vol. 2 no. 36, 2 August 1890, he wrote “Our work is that of pioneers. Let us do it faithfully and well, carrying our standard in the vein, bearing the motto: ‘Spiritualism Pure and Simple, Unalloyed by Theosophic Speculations.’”

Dean was living with his brother and family in Brookline, Massachusetts in 1910. He died two years later, in September of 1912 and was buried near his hometown in Vermont.