Slattery was born in Athens, Ohio on January 31, 1877. His parents were John A. Slattery (1847-1895) and Lena DeSteiguer (1854-1925). He attended the United States Military Academy where he graduated fifth in his class for the year 1900. He was assigned to the Philippines to work on bridges and roads as an engineer officer. He was assigned to Honolulu in November 1904 as the Honolulu District's first District Engineer for lighthouses.