Gallagher moved to New York in 1887, and worked in a restaurant. He was a part-owner of a restaurant on Carmine Street from 1894, and opened a restaurant named Broadway Gardens at 711 Seventh Avenue in 1911, which developed a reputation for good luck, based on the number of the address.[4] He also ran a restaurant in Times Square near 47th Street, where he introduced singing waiters,[4] a gimmick that had begun in the Bowery.[2]
Gallagher was well-known in the cabaret business, and was nicknamed "Little Billy,"[2] due to his height of 5 ft 5 in (1.65 m).[5] The New York Times described his cabaret as one that opened after the sun set and picked up business when other establishments closed at midnight, remaining open until dawn with a mix of "reputable people" and those "who had practical reasons for circulating after dark".[2] After New York City passed a curfew law requiring establishments to close at 1:00 am, Gallagher had at first observed the letter of the law, but ultimately developed a workaround under which patrons would be escorted out when the curfew went into effect and then would be welcomed back in after 15 minutes to continue their partying.[6]
During the Prohibition Era, Gallagher's establishments were among the first that were padlocked by U.S. Attorney Emory Buckner.[2] In 1923, Abigail Harding, sister of then-President Warren G. Harding, visited Gallagher's establishment in Times Square.[7]
In 1923, Gallagher took over management of the Monte Carlo restaurant at 51st Street and Broadway, with National Hotel Review calling him "one of the most popular cabaret owners in the city."[8]
Then a resident of Bayonne, New Jersey, Gallagher opened Club Lido in 1932, a nightclub on 52nd Street and Broadway, where he preferentially hired staff from Bayonne and other areas nearby in New Jersey.[9] Gallagher advertised a revue by Charles Elbey that advertised 16 blondes, brunettes, and redheads.[10]