Born to immigrants from Yugoslavia who settled in Johnstown, Dezelan served in the United States Navy during the Korean War. He played baseball during military service, and on the sandlots of Cambria County. He spent thirteen years umpiring in baseball before ending his career due a brain tumor.
His umpiring career was launched in a dentist's chair, according to his family. While being treated by his dentist, Richard Goldberg, Frank discussed his love of baseball, and Goldberg suggested that he become an umpire. Dezelan decided to heed the advice and was accepted into the Al Somers Academy of Professional Umpiring in Daytona Beach, Florida.
After graduating in 1958, Dezelan began his umpiring career in the Northern League that year and returned for the 1959 season. According to a story told by former and longtime major league umpire Bruce Froemming, Dezelan and his crew were scheduled to work a minor league doubleheader in which future Hall of Famer Earl Weaver was managing. Notorious for mixing it up with umpires, Weaver got into a pregame argument over some tree limbs that jutted into fair territory. Dezelan ejected the argumentative Weaver before the game even began.
Dezelan moved up to the South Atlantic League in 1960 and 1961 and to the Pacific Coast League from 1962 to 1963. He then worked in the Southern League in 1964 and 1965 and in the International League from 1966 through 1968. In addition, he would be an umpire in the National League for the last weeks of the season from 1966 to 1968, then full-time from 1969 to 1970. When the National League umpires began wearing numbers in 1970, Dezelan was assigned number 6.
During his career, Dezelan worked some Major League milestones involving some of its biggest star players. He was behind home plate when Willie Mays hit his 600th career home run in 1969. He was part of the umpiring crew at the 1970 All-Star Game that ended with Pete Rose colliding with catcher Ray Fosse at home plate, and also worked the inaugural game at Three Rivers Stadium during the 1970 season. In 1971 Dezelan had surgery to remove a brain tumor, which ended his career when he was 42. He survived six brain operations over the next four decades.
According to his son Daryl, Dezelan loved to work games with pitchers Sandy Koufax and Bob Gibson, because they worked so efficiently, and with outfielder Roberto Clemente because he never argued over calls, but just held his head high and played the game. Additionally, he worked with many of the most notable players and coaches in Major League history, including Sparky Anderson, Carl Yastrzemski and Willie Stargell, amongst others. Irene Dezelan, his wife of 52 years, noted that her husband made it to the big leagues when there were more U.S. senators than major league umpires. The couple raised five children, three daughters and two sons, in the Pittsburgh area.