Bloch served as head of the Jewish Division of the New York Public Library from 1923 to 1956. Under him, the Library developed one of the major Judaica collections in the United States. He arranged a number of exhibitions on Judaica at the Library. His bibliographical research into the history of Hebrew was published by the Library's Bulletin of the New York Public Library on multiple occasions. He founded the quarterly Journal of Jewish Bibliography in 1938 and served as its editor until 1943. He was appointed to the publication committee of the Jewish Publication Society in 1940, and in 1941 he was appointed to the editorial board of the Jewish Apocryphal Literature Series. He wrote On the Apocalyptic in Judaism in 1952, Of Making Many Books (an annotated list of books published by the Jewish Publication Society from 1890 to 1952) in 1953, and The People and the Book (a 300-year history of Jewish-American life) in 1954. His bibliography was collected by Dora Steinglass's 1960 book A Bibliography of the Writings of Joshua Bloch (1910–1958).[5]
In 1935, when pamphleteers Raymond J. Healey and Ernest F. Elmhurst claimed the Talmud commanded Jews to kill Christians for ritual purposes, Bloch discredited the blood libel accusation in the resulting trial by providing authoritative testimony that refuted the accusation. He served as rabbi in Rockville Centre, New York, and Lake Charles, Louisiana. He was trustee of the Union Free School District No. 5 in Nassau County for six years and of the Central High School District No. 2 for five years. From 1934 to 1937, he was president of the latter high school district.[2] By the end of his life, he was living in New Hyde Park, Long Island.[6]
Bloch died from a heart attack at Creedmoor State Hospital in Queens Village, Queens, where he was delivering a Rosh Hashanah sermon as a chaplain of the New York State Department of Mental Hygiene that operated the hospital, on September 26, 1957.[6]