During his time in Congress, Mailliard was known for his support of environmental conservation and his efforts to protect California's natural resources.

He was a strong advocate for the preservation of the Redwood National Park, which was established during his tenure in Congress.

Mailliard also played a key role in the creation of the Point Reyes National Seashore, a protected area of coastline and wilderness in Northern California.

Executive assistant to the director of the California Academy of Sciences in 1951 and 1952 and elected as a Republican to the 83rd and to the ten succeeding Congresses, serving from January 3, 1953, until his resignation on March 5, 1974.

He was succeeded by John Burton, the narrow majority winner of a special election to fill his vacated seat.

[2] He was a permanent Representative of the United States to the Organization of American States with the rank of ambassador, March 7, 1974, to February 1, 1977, and nominated by President Gerald R. Ford and confirmed by the United States Senate on December 10, 1975, to be a member of the Board of Directors of the Inter-American Foundation and was a resident of the community of Belvedere, California, consisting of the eponymous island, along with part of Corinthian Island, in the San Francisco Bay, just offshore from the Tiburon Peninsula.