During the Civil War, he served as a clerk to the provost marshal in Salem, Oregon from 1863 to 1864.

In 1893 he founded and was elected treasurer of the Elmwood Mills, manufacturers of shoe laces, and held that position until 1912, when he was elected president of the International Braid Company, which made boot, shoe and corset lacings.

He maintained the International Braid Company in a profitable state, and occupied a leading position among business leaders in Rhode Island.

Dennis was interested in historical research, and had a valuable collection of early Colonial relics and furniture.

Lieutenant Governor Dennis died in 1920 and was buried in Swan Point Cemetery in Providence.