Mansfield was the first American officer who was in headquarter of Chetnik leader Draža Mihailović.

[2] Mansfield was nominated by President Lyndon B. Johnson on June 13, 1966, to a seat on the United States District Court for the Southern District of New York vacated by Judge John M. Cashin.

He was confirmed by the United States Senate on June 29, 1966, and received his commission the same day.

[2] Mansfield was nominated by President Richard Nixon on April 26, 1971, to a seat on the United States Court of Appeals for the Second Circuit vacated by Judge Leonard P. Moore.

[2] His service was terminated on January 8, 1987, due to his death from a stroke while on vacation in Christchurch, New Zealand.