In New York, Hall’s connections secured him a partnership with distinguished lawyers Aaron Vanderpoel and Augustus Brown and a position as assistant to district attorney Nathaniel Bowditch Blunt.  When Blunt died in 1853, Hall was elected district attorney on the Whig ticket.  He was subsequently reelected several times, serving until 1869 except for the 1859-61 term.  An industrious worker and lover of courtroom drama, he prosecuted 10,000 cases, including several publicity-generating murder trials.  Hall switched his political affiliation from Whig to Republican to Democrat, aligning himself with the city’s Tammany Hall Democratic machine by 1864.  He edited Tammany Hall’s news organ, the Leader, until 1871.  He became wealthy as his prominence attracted clients to his law practice.

“Boss” William Tweed handpicked Hall to be Tammany’s mayoral candidate in 1868.  Hall won by a landslide, and was reelected in 1869 and 1870 (in the latter to a two-year term).  Tweed used his position as state senator to drive through the legislature a reformed city charter that augmented mayoral authority.  It established the mayor and two other city officials (also Tammany associates at the time) as a Board of Special Audit that certified city expenditures.  The change in political power resulted in numerous projects expanding the city’s infrastructure as well as corruption in the guise of inflated payments to contractors and kickbacks to government officials. 

The New York Times and Harper’s Weekly exposed the graft and effectively pursued Hall and other members of the “Tweed Ring” out of office.  Hall was tried three times (1871-1873) for malfeasance, but it could not be proved that he intended to defraud the public nor that he benefited financially from approving monetarily bloated contracts.  The first case against him was ruled a mistrial, the second ended with a hung jury, and the third in acquittal. 

Despite the “not guilty” verdict in the courtroom, the scandal killed Hall’s political career and brought hardships to his personal life.  He was forced to resign from his law firm and the socially prestigious Union Club, and relations with his wife became strained.  For the next few years he ventured rather unsuccessfully into playwriting, acting, lecturing, and journalism.  In 1883, James Gordon Bennett Jr. hired him to be the London correspondent for the New York Herald.  In London, Hall resumed the practice of law, socialized with artists, and remarried after the death of his first wife.  In 1892, he returned to New York City, writing occasionally for newspapers until his death on October 7, 1898.