By 1865 he had moved into banking and specialized in selling municipal, state, and government bonds. Relying on his bank president father's connections, his political contacts, and his own acumen, Yerkes gained a name for himself in the local financial and social world. However, while serving as a financial agent for the City of Philadelphia's treasurer, Joseph Marcer, Yerkes risked public money in a large-scale stock speculation. This speculation ended calamitously when the Great Chicago Fire sparked a financial panic. Left insolvent and unable to make payment to the City of Philadelphia, Yerkes was convicted of larceny and sentenced to thirty-three months in Eastern State Penitentiary.