Born in Sicily, Ninfo immigrated to the United States in 1899. For the next decade, Ninfo worked to organize Italian workers and lead in the garment industry's major early strikes. Except for a period (1903-1906) when he worked to organize Italian craft workers in New York, Philadelphia, and Boston for the American Federation of Labor, Ninfo worked with the ILGWU.
In 1902, Ninfo joined Local 9, the New York Cloak Finishers' and Tailors' Union. In 1906, when he returned from organizing for the AFL, he served as a member of the executive board of Local 9, and in 1908, he was both a delegate to the Cloak Joint Board and a general organizer for the ILGWU.
Ninfo was deeply involved in the dressmakers' strike (1909) and the cloakmakers' strike (1910), also known as "The Uprising of the 20,000" and "The Great Revolt," and after the Protocol of Peace, he served as a business agent for the New York Cloak Joint Board and manager of Local 48.
Ninfo was elected vice president (1916) and then for a brief period of about four months, between Benjamin Schlesinger's resignation and Morris Sigman's election, he served as Acting President of the ILGWU (1923). After Sigman's election, Ninfo became first vice president (1923-1934) and later (1936), manager of Local 145 in Passaic, New Jersey.
Ninfo left the ILGWU when he was elected to the New York City Council on the American Labor Party ticket in 1937. He served on the city council until 1943.