 In 1915, Manton was attorney for Charles Becker, the New York City police officer who was convicted and executed in the Rosenthal murder trial.Manton was nominated by President Woodrow Wilson on August 15, 1916, to a seat on the United States District Court for the Southern District of New York vacated by Judge Charles Merrill Hough.He was confirmed by the United States Senate on August 23, 1916, and received his commission the same day.