When Morton was appointed United States Secretary of the Interior by U.S. President Richard Nixon in 1971, Mills won the special election to succeed him in Maryland's 1st District in the 92nd Congress.

[4] It was reported that he had been depressed following the death of three of his Congressional aides in a 1972 car accident, and by the fact that his mentor and predecessor, Rogers Morton, was suffering from cancer.

[4] However, five days before his death, it was revealed that Mills had received an undisclosed $25,000 gift from the finance committee of President Richard Nixon's re-election campaign during the 1971 special election.

[5] While Mills initially stated that he had done nothing wrong, he began worrying that the campaign contribution would destroy his political career.

[5] Despite his concern, Maryland authorities claimed soon after his death that he may not have broken the new state campaign finance law, which did not come into full effect until July 1971, two months after his special election.