In 1880 he ran successfully for Nevada's At-Large seat in the United States House of Representatives. He was reelected in 1882 and served from March 4, 1881, to March 3, 1885. In his second term Cassidy was chairman of the Committee on Pacific Railroads. He was an unsuccessful candidate for reelection in 1884. In 1886, he was appointed a national bank examiner for Nevada, Utah, California, and Colorado, and he served until 1890.
