During his tenure in the House of Representatives, he supported multiple environmental bills and was a staunch opponent of American involvement in the Vietnam War.

Celeste Domenico Roncaglio was born on March 23, 1916, in Rock Springs, Wyoming, to Frank and Ernesta Roncalio, Italians who had immigrated to the United States in 1903.

[4] He enlisted into the army following Pearl Harbor and during World War II he fought at the Battle of Gela and was later awarded a Silver Star for gallantry in the Normandy invasion on Omaha Beach.

[6] While in college, he was elected as president of the student body, joined the Young Democrats, and Senator Joseph C. O'Mahoney offered him a job in Washington, D.C.[7] In 1947, he graduated from the University of Wyoming with a law degree.

Later that year Governor Milward Simpson proposed a civil rights bill that Roncalio had drafted after seeing a black couple being removed from a restaurant.

[10] As Chairman of the Wyoming delegation to the 1960 Convention, he cast the fifteen votes which gave John F. Kennedy the minimum amount needed to win the Democratic presidential nomination.

[12] Kennedy later appointed him as chairman of the International Joint Commission on Water Rights between the United States and Canada in 1961 and served until 1964.

[14][15] Upon taking office he praised President Johnson for his state of the union speech and called it the "20th century restatement of the constitutional principles on which this nation is founded".

[23][24][25] In April 1969, William A. Norris Jr., Wyoming's Democratic national committeeman, announced that he would resign and on May 5, 1969, Roncalio was selected to replace him by acclamation after Joe Stewart, the only other candidate, withdrew two days before.

[29][30] Although he did not endorse him, Roncalio stated that Senator Edmund Muskie was the most balanced candidate during the 1972 Democratic presidential primaries, but later voted for George McGovern at the national convention in Miami Beach, Florida.

[50] On March 30, 2003, Roncalio died of congestive heart failure at the Life Care Center in Cheyenne, Wyoming and was buried in Mount Olivet Cemetery.

Governor Dave Freudenthal, former Governor Mike Sullivan, Senator Craig L. Thomas, State Chief Justice William U. Hill, former Secretary of State Kathy Karpan, and other Wyoming political figures attended his funeral and a letter from Senator Ted Kennedy was read at the funeral.

[53] In 1966, he proposed that every window on commercial airplanes should be turned into emergency exits and attempted to have the Federal Aviation Agency support his idea, but he was unsuccessful.

[63] In 1966, he supported an effort to remove funding for the House Un-American Activities Committee and created a resolution demanding that France pay back its $6 billion in war debts to the United States.

In 1969, he criticized Richard Nixon's Peace with Honor plan as a "phony promise" and that the United States had failed in Vietnam and should withdraw its soldiers.