Sickles then studied law at New York University. Afterwards he became involved in politics and was a key figure in the Tammany Hall political machine. He also served as their legal counsel. Through political influence he became the Corporate Council for New York City, Secretary of the United States Legation in London and later a US congressman from New York. In 1852 he married Teresa Bagioli. At the time he was 33 years old and she was 15. Both families were opposed to the marriage at the time. Only a few years later, living in Washington, D.C. as the wife of a congressman, she was described as "more like a schoolgirl than a polished woman of the world" with a "sweet, amicable manner".