Kelly was able to amass a vast fortune estimated at $800,000 ($17,400,000 in current dollar terms) by 1867 by both ethical and questionable means.

In addition, after having his methods questioned and his title insulted by New York City Mayor William Havemeyer, Kelly responded with a lawsuit for libel.

Kelly, in response to anti-Catholic sentiment, was driven to politics and became a champion of Catholic and immigrant causes in the 1840s.

[4] Kelly joined the influential Tammany Society and the next year he was elected alderman; until his death, he was active as a Democratic politician.

[5] During Kelly's time as sheriff, his wife and children died and he left New York for an extended overseas trip.

[4] By 1874, Kelly was in control of Tammany Hall, and for the next decade, he was able to determine the course of New York City elections.

[5] In 1876, Kelly succeeded Andrew H. Green, appointed by Mayor William Wickham,[1] as Comptroller of New York City.