Born in Kingston, Upper Canada (Ontario), Dennis was educated at Victoria College in Cobourg, and, after apprenticing with Charles Rankin, became a qualified land surveyor in 1842. He surveyed several towns along the planned routes of the Grand Trunk and Great Western Railways, as well as several Indian reserves. In 1855 he began a military career lasting nearly two decades, tainted, however, by an investigation of his leadership at Fort Erie during the Fenian invasion of 1866. In 1871 Dennis was appointed Canada’s first surveyor-general, overseeing the mapping and division of the western prairie lands, and further opening the way to railroads and settlements. From 1878 to 1881, he served as the Deputy Minister of the Interior.
Dennis lived for a time in Toronto, and in 1854 he created and registered plans for the subdivision of Rose Park (the southern section of what is now Rosedale) at the northeastern corner of Bloor and Yonge Streets. The land formerly comprised the estate and residence known as Rosedale Villa, built in 1821 by John Small, and purchased in 1824 by politician and entrepreneur William Botsford Jarvis, who sold most of the property in 1853 for development. One of Canada’s early examples of a curvilinear subdivision, Dennis’ plan established 62 irregular lots disposed around four winding streets—a design well suited to the natural ravines in the area. In 1882 Dennis was made a Companion of the Order of St. Michael and St. George. He died three years later near Ottawa, and is buried in Kingston.