Denis O’Brien was born March 13, 1837 in Ogdensburg, son of John and Katherine Maloney O’Brien, and was educated in St. Lawrence County schools and seminaries. He began the study of law in Ogdensburg and was admitted to the Bar in 1861. In November of that same year, he established a law office in Watertown.
In 1869, he was elected Alderman of his ward in the City’s first year of incorporation. He served four consecutive terms as Alderman and then was elected Mayor of the City in November 1878, serving a one-year term.
In 1880, he succeeded the late James F. Starbuck as a member of the Democratic State Committee and held that position for four years. He was elected New York State Attorney General in 1883 and served in that capacity until 1888. The following year, he was elected by a large majority to the position of Appellate Court Judge and, after serving the constitutional term of fourteen years, was re-elected in 1903.
He married Margaret McCahill and the couple had six children: Edward D. O’Brien, John F. O’Brien, Louise O’Brien and Gwendoline O’Brien Berg as well as Anna O’Brien and Charlie O’Brien who died in infancy.
He died in the City of Watertown on May 18, 1909 at the age of 72. He is buried in Calvary Cemetery.
