He left Congress to become the Queens County District Attorney and was defeated for re-election after being indicted on Federal corruption charges.

[3] In July 1947 Quinn was appointed by President Harry Truman as an Assistant United States Attorney General for the Criminal Division and was confirmed in September.

In the post, he worked on the post-war cases of World War II propagandists Iva Toguri D'Aquino, better known as Tokyo Rose and Robert Henry Best.

He defeated Ross in a rematch in 1950; but ran successfully for Queens County’s District Attorney in 1951 and resigned from Congress in December.

The judge ruled that prosecutors had presented no evidence that Quinn was aware the legal fees in question were from cases that took place during his time in Congress.