Born in Coraopolis, Pennsylvania, DeMascio entered in the United States Naval Reserve during World War II, from 1943 to 1946.

On June 14, 1971, DeMascio was nominated by President Richard Nixon to a seat on the United States District Court for the Eastern District of Michigan vacated by Judge Theodore Levin.

DeMascio was confirmed by the United States Senate on July 22, 1971, and received his commission the same day.

In 1975, DeMascio took over the complicated desegregation case of Milliken v. Bradley from Judge Roth.

For five years DeMascio worked to enforce desegregation in Detroit schools and a number of court-ordered improvements.