In 1871 he became surveyor-general of DOMINION LANDS, ie, the person chiefly responsible for mapping the Prairie West, and was deputy minister of the Interior Department 1878-1881.
His son, J.S. Dennis Jr, followed a similar career as a surveyor in the West in 1872, as commander of a militia unit against Riel at BATOCHE in 1885, and as inspector of surveys 1887-94. J.S. Dennis Jr wrote a history of the Dominion lands survey 1869-89 (when his father had proposed a better system than the US standard eventually adopted) and ended his career as chief of the CPR Department of Natural Resources in charge of lands and irrigation.