[1] Born in St. Andrews, County of Antigonish, Nova Scotia, the son of Donald McDonnell (Garanaich) and Mary Macdonald,[2] McDonnell studied law in Antigonish with William Alexander Henry.

After being admitted to the bar in 1862, he practiced law in Port Hood, Nova Scotia.

He married Annie, the daughter of Peter Smyth in 1866 and was named Queen's Counsel the following year.

In 1872, he was elected to Parliament as a member of John A. Macdonald's Conservative caucus.

[1] He was later appointed by the Laurier Government an Inspector of Customs for Eastern Nova Scotia.