[1][2] His father, also called James Sandilands Douglas, was the publican of Wain's Hotel in Dunedin, and his mother was Agnes Fortune Douglas (née McFadyen), whose father Hugh McFadyen was the first town clerk of North East Valley Borough.

[1][3][4] On 18 April 1900, Douglas married Catherine Mackie at the Leith Street Congregational Church in Dunedin.

[6] At the 1911 general election, he stood as an independent candidate for Dunedin North, but again placed second, being defeated by George Thomson.

[10] Douglas was a founding member of the Otago Master Plumbers' Association serving as secretary from 1895 to 1898, and then as president from 1899, retiring in 1933.

He chaired the provisional committee that launched the New Zealand and South Seas International Exhibition, which was held in Dunedin in 1925–1926.

[1][8] In 1891, Douglas joined the Dunedin City Guards as a private; he rose through the ranks, being commissioned as a lieutenant in 1899, and was promoted to captain in 1906.