Daniel O'Neill was predominantly self taught as a painter. Like his father, he was an electrician by trade and briefly attended classes at the Belfast College of Art before working and studying under the artist Sidney Smith. His romantic, expressive style emerged early in his work, showing common themes of love, life and death. His first exhibition was in 1941 at the Mol Gallery in Belfast. O'Neill's career as a painter was launched after being awarded a gallery contract with Dublin art dealer, Victor Waddington, in 1945. This provided him with a steady source of income and allowed him to paint full time free of any other obligations. He exhibited and regularly sold his work at the Waddington Gallery in Dublin and was also part of group exhibitions with Gerard Dillon at the Contemporary Painters Gallery.
In 1949, O'Neill travelled to Paris to study the work of Georges Rouault, Maurice de Vlaminck and Maurice Utrillo, who greatly inspired him. The pieces he produced after this visit to Paris are considered by some as his finest work.
O'Neill continued to paint and exhibit regularly across Great Britain and Ireland for many years, however, after the closure of Waddington's in Dublin he found it increasingly difficult to support himself financially. He lived and painted in London from 1958 to 1971. Gallery owner, George McClelland invited O'Neill to have a solo exhibition in his gallery in 1970, O'Neil's first in eighteen years. Titled, Recent Paintings, the exhibition was a huge success-every painting sold. As luck would have it, this success was short lived after civil unrest in Belfast broke out in the early 1970s and had a negative impact on the McClelland Gallery.
O'Neill returned to Belfast in 1971 where he remained until his death in 1974.
His work is represented in many collections including the Ulster Museum, Queen's University Belfast and the Hugh Lane Municipal Gallery, Dublin. A retrospective of O'Neill's work was held at the Belfast Museum and Art Gallery.