In 1947, Roncalio graduated from the University of Wyoming.
During World World War II, he served in the United States. Army and was awarded a Silver Star for gallantry in the Normandy invasion. He served as the prosecuting attorney for Laramie County from 1950 to 1956. In 1957, he was elected as chairman of the Wyoming Democratic Party.
Roncalio also was chosen as a delegate to the Democratic National Convention in 1956, 1960, 1964, and 1968.
In 1964, he narrowly defeated Republican Republican Roncalio gave up the seat two years later to become the Democratic nominee for United States Senate.
He was edged out in that race by Republican Governor Clifford P. Hansen. In 1970, when Wyoming"s House seat became open again, Roncalio ran again for his old job, winning by just 608 votes.
He was re-elected narrowly in 1972, but by a wider margin in 1974, when he defeated Thomas F. Stroock, and again in 1976.
He casually announced his plans to leave politics at a 1977 football game, and did not run for re-election in 1978. He was succeeded by Republican Dick Cheney. Roncalio then returned to Wyoming, where he served as Special Master in Wyoming"s Big Horn adjudication of Indian Water Rights until 1982.
Roncalio died of congestive heart failure in 2003, and is interred in Cheyenne.
The post office in Rock Springs, Wyoming, is named in his honor. As of 2015, he was the last Democrat to have served Wyoming in Congress.