In 1950, he began working as editor of the "Wyoming Labor Journal". 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. 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. He also served as a delegate to the 1956, 1960, 1964, and 1968 Democratic National Conventions. In 1958, it was speculated that he might be a possible Attorney General appointee, but on December 3, 1958, he stated that he did not want to be appointed to the office.