He was a founding member of the committee which established the Glasgow Celtic football club,[2] but is probably better known for the William McKillop Cup, which he donated to Armagh Gaelic Athletic Association.
William and his brother had moved to Glasgow, where in time they set up a licensed grocers, and were successful enough to buy out other businesses including the Royal Restaurant.
William McKillop was asked to stand as the nationalist candidate for North Sligo at the 1900 general election, and readily agreed.
For reasons that are unclear, McKillop switched seats at the 1906 general election, from North Sligo to South Armagh, where he was returned unopposed.
He remarried in 1908, to Rose Dalton, a sister-in-law of the Willie Redmond MP from Orange, New South Wales, Australia.