In The Gangs of New York, Herbert Asbury had a tendency to shoehorn all famous crimes and criminals into association with the city’s infamous gangs, despite there being no evidence for those connections. An example can be found in his contention that Happy Jack Mulraney was a member of the Gophers gang. In Chapter XII, Section 2 he writes:
“Still another Gopher of distinction was Happy Jack Mulraney, so called because he always appeared to be laughing. However, the smile was caused by a partial paralysis of the muscles of his face. In reality Happy Jack was a verjuiced person and very sensitive about his deformity; when his chieftains wished to enrage him against an enemy they told him that slighting remarks had been made about his permanent grin. Happy Jack was finally sent to prison for the murder of Paddy the Priest, who owned a saloon in Tenth avenue and was a staunch friend of Happy Jack’s until he asked the gangster why he did not laugh on the other side of his face. Happy Jack then shot him and for good measure robbed the till.”
Hundreds of newspaper articles appeared about John “Happy Jack” during his arrest, conviction, appeals and eventual execution in the electric chair in May, 1913. Not one of them mentioned the Gophers, or a facial deformity. When Mulraney was sentenced for Patrick “Paddy the Priest” McBreen’s murder, he was sent to Sing Sing, where the prison intake records meticulously recorded every prisoner’s physical features. Nothing was mentioned about a facial deformity; instead, his features were characterized as “regular.”
The victim, Paddy the Priest, earned his nickname through acts of kindness and charity, despite being a saloon operator. There is no evidence he knew Happy Jack before the crime. Instead, witnesses testified that Mulraney and three others had been searching for a clothing store to rob that evening, but were frustrated by iron security bars. Mulraney and a companion walked down the block and went into McBreen’s bar, where Paddy was counting receipts at the till. Mulraney and his partner pulled guns. McBreen thought they were joking, and reached behind his back to untie his apron. Mulraney thought he was reaching for a weapon, and shot him three times.
The murder occurred on October 4, 1911. John Mulraney had just been released from Auburn State Prison a month earlier; he had been sent there in May, 1907 for Burglary (his third offense since a year earlier, 1906). One witness identified Mulraney from looking at the NYPD Rogue’s Gallery photographs, and pointed to Mulraney because he looked so mean. A grainy reproduction of the mug shot appeared in the March 15, 1914 New York Herald, alongside a full-page article describing how detectives found evidence and tracked down their man. The photo is dark, but definitely shows a man without a smile:

However, Happy Jack earned his nickname because he did smile often, and inappropriately. He grinned during his trial, laughed when sentenced to die, and (by some accounts) had the remnants of a smile on his lifeless face after being electrocuted. Before he died, he told prison reformer Madeleine Zabriskie Doty that his life was a joke. By Doty’s account, Mulraney’s life had been one of almost constant street life and crime. He did not appear to have much cunning, but that may have been an act. While on Death Row, he succeeded in gaining an appeal hearing and a stay of execution by presenting four or five different versions of who committed the murder and how he was not present as the scene. Several of his defense witnesses soon admitted to perjury (including his mother!), and in the end of own defense lawyer threw in the towel and refused further motions.
Those who cling to the idea that Mulraney might have inspired the Joker are mistaken. If any one figure inspired the comic-book villain, it was Victor Hugo’s protagonist Gwynplaine, in The Man Who Laughs. Hugo’s work was made into a Hollywood movie in 1928–the same year that The Gangs of New York was published.
Still, some may point to this clipping from 1903:

However, cold water (if not acid) can be thrown on this by pointing out that different articles gave the man’s name as John Mulvaney, Joseph Mulraney, and James Mulvaney; and that he was a stock clerk by profession; he was permanently blinded in one eye and his face was severely burned.