Born in Chicago, he attended parochial schools and St. Ignatius College (now known as Loyola University Chicago). He was employed as a secretary with the building departments of the city of Chicago in 1905, and graduated from Illinois College of Law at Chicago in 1908. He was admitted to the bar that year and commenced practice in Chicago, and was a member of the Illinois College of Law faculty from 1909 to 1912. He was superintendent of public service of Cook County, Illinois from 1910 to 1912 and was secretary of Webster College of Law in Chicago from 1912 to 1914. He was counsel for the Lincoln Park Commission in 1933 and 1934 and for the Chicago Park District, 1934 to 1936. He was chairman of the Illinois Commerce Commission from 1936 to 1939, and was appointed on April 14, 1939 as a Democrat to the United States Senate to fill the vacancy caused by the death of James Hamilton Lewis and served from April 14, 1939, to November 21, 1940, when a duly elected successor qualified. Slattery was an unsuccessful candidate for election to fill the vacancy, and resumed the practice of law in Chicago. He died at his summer home at Lake Geneva, Wisconsin and was interred in Calvary Cemetery, Evanston, Illinois.
- United States Congress. "James M. Slattery (id: S000478)"  . Biographical Directory of the United States Congress.