John Charles O'Neill (9 March 1834 – 8 January 1878) was an Irish-born officer in the American Civil War and member of the Fenian Brotherhood.
He was credited as being a daring fighting officer, but believed he had not received due promotion, which led to a transfer to the 17th United States Colored Infantry as captain.
The following day, north of Ridgeway, Canada West, O'Neill's group encountered a detached column of Canadian volunteers, commanded by Lieutenant-Colonel Alfred Booker (mainly formed of the Queen's Own Rifles of Toronto and the 13th Battalion of Hamilton).
With overwhelming numbers of Canadian forces closing in, O'Neill oversaw a successful evacuation on the night of 2–3 June back to US territory.
After the Battle of Trout River ended in a disorganized rout, O'Neill was arrested by United States Marshal George P. Foster and charged with violating neutrality laws.
With the latter, and without the backing of the bulk of the Fenians, he led an attack on the Hudson's Bay Company's post at Pembina, Dakota Territory, on 5 October 1871.