In 1890, he married Alice Jones, and they had two children. In 1899, he was a member of the New York State Board of Health.
Cassidy was a member of the New York State Senate (40th D.) from 1905 to 1908, sitting in the 128th, 129th, 130th and 131st New York State Legislatures. In 1908, Cassidy was voted down by the 41st senatorial district convention, and ran as an Independent for re-election, but was defeated. Afterwards he resumed the practice of law in Watkins.
He died on January 15, 1911, at his home in Watkins, after having been ill for years, and "bedridden for the last few months".
