Stello was born in Boston and lived in an orphanage until age 12. At that point, he went to live with a foster family on a rural Massachusetts farm. Stello joined the United States Air Force and was an airman first class. He was introduced to umpiring while in the air force stationed in Tokyo. Stello umpired local baseball for a year before attending an umpire school and graduating at the top of his class.
Stello was married to stripper and actress Liliana Wilczkowska, better known as Chesty Morgan, from 1974 to a 1979 divorce. After the split, they stayed in contact until his death.