He was elected by the electorate of Stanley to the South Australian House of Assembly to fill the vacancy left by the resignation of Edward William Hawker, and served from June 1889 to September 1896, when he died.
[4] His colleagues were Charles Kimber, followed by John Miller then Hawker, who had been studying mineralogy in England.
One of the first debates in which he made his presence was Sir John Downer's Divorce Bill, to which he had a strong opposition.
His administration of the Land Acts was marked by conspicuous ability, but his name was principally associated with the Village Settlements scheme of 1894.
He was buried at Sevenhills on 22 September 1896 Gillen married Mary Gertrude Cousins (c. 1860 – 22 January 1929) of Clare on 24 May 1881.