O'Neill, of Canadian extraction, grew up in Chicago, Illinois and Detroit, Michigan. He and his father moved to Birmingham when he was fifteen. He founded the John W. O'Neill Company, which operated "The Fair", a cookware and gift shop. He helped found the Commercial Club of Birmingham and the Birmingham Board of Trade, which he served as president. He was a delegate to the Convention which drafted the Alabama Constitution of 1901 and was exalted ruler of Birmingham Elks Lodge No. 79, representing the city at the Grand Lodge meeting in Salt Lake City, Utah.
O'Neill defeated incumbent C. T. Rambow to represent the Second Ward on the Birmingham Board of Aldermen in 1899. He resigned after his second term, but returned in 1905 and remained in office until 1909. He later was elected vice president of the Alabama Good Roads Association.
O'Neill, a Catholic, built a private chapel at his home and purchased a 1586 painting, Madonna and Child Enthroned with Saints Jude, Simon Zelotes, and the Young John the Baptist by the Florentine artist Giovanni Maria Butteri as the altarpiece. The painting is now in the collection of the Norton Museum of Art in West Palm Beach, Florida.
- Dubose, Joel Campbell (1904) Notable men of Alabama: Personal and Genealogical 2 Volumes. Atlanta, Georgia: Southern Historical Association
- Madonna and Child Enthroned with Saints Jude, Simon Zelotes, and the Young John the Baptist, provenance at Robert Simon Fine Art