Upon graduating, he studied law in the offices of Benjamin H. Brewster, and was admitted to the bar in 1849.

He was immediately successful, and proved popular with the Moyamensing Hose Company,[Note 1] and was elected to the state House for the 1851 term.

By 1856, Cassidy was already considered a local political boss, and his influence helped elect Richard Vaux city mayor.

[3][4] During the Civil War and afterwards, he was a staunch supporter of Union and federal coercion.

His support helped Robert E. Pattison become Governor—the state's only Democratic governor between the Civil War and the Great Depression—and Pattison in turn appointed Cassidy state Attorney General.