McCornack was born in Chicago, Illinois on January 22, 1875. He attended Chicago's Englewood High School before entering Dartmouth with the class of 1897. At Dartmouth, McCornack played football and baseball and was the captain of the football team in 1895 and 1896. McCornack graduated from Dartmouth in 1897 an earned an LLB from Northwestern in 1899. He worked as a lawyer for the Interstate Commerce Commission. McCornack died at his home in Chicago on June 30, 1939.