O’Neill is born on November 15, 1847 in County Kilkenny. The family emigrates to the United States and settles in Buffalo, New York. In 1857 they move to Cincinnati, Ohio where James is apprenticed to a machinist.
At the age of 21, he makes his stage debut in a Cincinnati production of Dion Boucicault‘s The Colleen Bawn (1867). Also in 1867, he has a minor part in Edwin Forrest‘s production of Virginius, and then joins a travelling repertory company. By the age of 24 he has already established a reputation among theater managers as a box-office draw, particularly with the ladies. But he is also working doggedly at his craft, ridding himself of all vestiges of brogue and learning to pitch his voice resonantly. He is considered a promising actor, quickly working his way up the ranks to become a matinée idol.
On June 14, 1877, while in New York City, O’Neill marries Mary Ellen Quinlan, daughter of Thomas and Bridget Quinlan. In the fall of 1877, three months after his marriage, a woman by the name of Nettie Walsh sues O’Neill, claiming that he had married her five years earlier, when she was only 15, and that he is the father of her three-year-old son. Nettie Walsh loses her case and the publicity, although it wounds his bride, enhances his reputation as a romantic leading man.
As early as 1875, while a stock star at Hooley’s Theatre in Chicago, O’Neill plays the title role in a stage adaptation of Alexandre Dumas‘ The Count of Monte Cristo. In early 1883 he takes over the lead role in Monte Cristo at Booth’s Theater in New York, after Charles R. Towne dies suddenly in the wings after his first performance. O’Neill’s interpretation of the part caused a sensation with the theater-going public.
O’Neill soon tires of the Count and his lines come out by rote and his performances become lackadaisical. Monte Cristo remains a popular favorite so he continues the role on tour as regular as clockwork. He goes on to play this role over 6,000 times.
In the middle of 1920 O’Neill is struck by an automobile in New York City and taken to Lawrence+Memorial Hospital in New London, Connecticut. He dies, at the age of 72, on August 11, 1920 of colorectal cancer at the family summer home, the Monte Cristo Cottage, in Connecticut. His funeral at St. Joseph’s Church is attended by, among others, O’Neill’s sister, Mrs. M. Platt of St. Louis and Edward Douglass White, Sr., Chief Justice of the United States Supreme Court. O’Neill is buried in St. Mary’s cemetery.