A farmer and grazier by trade, he was also one of the first European explorers of the Esperance district as well as a councillor and chairman on the Toodyay and Northam Road Boards for many years.
In 1864, he and his brother became the first European explorers to reach the Esperance district, and they opened up a stock route to the markets at Perth.
By 1866, he had become the manager of the family property, Buckland, at Northam, and on 5 March 1867, he married Mary Bussell, with whom he was to have three sons and four daughters.
In 1871, the Dempsters acquired the cutter Gypsy which was used to sail wool to Adelaide, stores and stock from Fremantle and hunt seals at the islands off Esperance.
In 1886, after selling the steam mill and store to George Throssell, Dempster purchased the 1,560 acres (630 ha) Springfield estate near Northam, and farmed at Wongamine.