Kearney was born in Oakmount, County Cork, Ireland. In census and voter registration records his birth year is listed as either 1846,[5] 1847[6] or 1848.[7] The second of seven sons, he left home after his father died when he was just 11 years old.[8] He became a cabin boy on the clipper ship Shooting Star, and by his own account he "circumnavigated the globe."[4] In 1868 he arrived in the United States and married an Irish woman named Mary Ann Leary.

He thought of himself as a "workingman's advocate",[10] although he remained highly critical of unions throughout his life and frequently denounced strikes.[4]