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. His family later removed the G in their last name; he was known by the diminutive "Celestino", so was given the nickname "Tino" as a child, which later became Teno. In 1933 he earned his barber's license and after high school he worked as a reporter for the Rock Springs Rocket-Miner. 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.
In 1940 he started editing the Wyoming Collegiate features which were published by the Casper Tribune-Herald newspaper. 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. In 1947 he graduated from the University of Wyoming with a law degree.
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.
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. Following Senator-elect Edwin Keith Thomson's death Kennedy asked Governor John J. Hickey to appoint Roncalio to fill the Senate vacancy, but he chose to instead appoint himself. Kennedy later appointed him as chairman of the International Joint Commission on Water Rights between the U.S. and Canada in 1961 and served until 1964.
On April 28, 1964, he announced that he would run for the Democratic nomination for Wyoming's at-large congressional seat and in the general election he narrowly defeated incumbent Representative William Henry Harrison with the coattail effect of President Lyndon B. Johnson's victory in Wyoming during the presidential election helping him.
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". During the 89th session he served on the Interior and Veterans Affairs committees. On June 15, 1966 he formally announced that he would run for Senate instead of seeking reelection, but was defeated by Governor Clifford P. Hansen.
After losing the Senate election Roncalio filed multiple affidavits for land claims around the Snake River and it was publicly revealed in 1972 that his land claims were estimated to hold $7 billion worth of gold.
In 1967 he was asked to run for the House again in the 1968 election, but chose not to. During the 1968 Democratic presidential primaries he supported Senator Robert F. Kennedy and was a member of his staff. When Roncalio heard about Robert Kennedy's assassination he stated that "I can't think of anything appropriate newsworthy or decent to say". After Kennedy's death he supported the anti-Humphrey movement at the national convention. 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.
In 1969 he stated that he would not run against incumbent Senator Gale W. McGee in the Democratic primary and stated that he would either run for governor or house. On June 23, 1970, he announced that he would seek the Democratic nomination for Wyoming's at-large House seat. In the primary he easily defeated state representative and future governor Edgar Herschler and in the general election narrowly defeated Harry Roberts by 608 votes.
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. Roncalio stated throughout 1971 that he would not run against Clifford P. Hansen for Senate again and on July 20, 1972 he filed to run for reelection and was reelected by a similar margin in the general election against Bill Kidd. On June 28, 1974 he announced that he would seek another term and defeated state senator Thomas F. Stroock by a wide margin. He was reelected in 1976 after defeating Larry J. Hart.
During the Watergate investigation he remained uncommitted until after the "smoking gun" tape was released and supported his impeachment. He stated that an impeachment trial should happen after a new vice president was confirmed after Spiro Agnew's resignation and in 1973 he voted in favor of House Minority Leader Gerald Ford's appointment as vice president. Following Nixon's resignation and Ford's accession to the presidency Roncalio voted in favor of Nelson Rockefeller's appointment as vice president.
On September 17, 1977, he announced that he would not run for reelection while at a University of Wyoming football game and stated that he would not run for governor giving his support to former state representative Edgar Herschler. In the 1978 election former White House Chief of Staff Dick Cheney easily won to succeed Roncalio and Roncalio resigned early on December 30, 1978.
He returned to Wyoming, where he served as Special Master in Wyoming's Big Horn adjudication of Indian Water Rights until 1982. In 1980 he endorsed Jim Rogers' house campaign, but he was defeated in a landslide by Dick Cheney and in 1982 endorsed Rodger McDaniel's unsuccessful Senate campaign. In 1986 he served as co-chair of Kathy Karpan's 1986 Secretary of State campaign and in 1990 he donated $1,000 to Pete Maxfield's unsuccessful House campaign against Craig L. Thomas.
In 1980 he explored for gold around Snake River in Teton county. In 2002 the post office in Rock Springs, Wyoming was named in his honor. 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 it.