McCallany was born September 3, 1963, in New York City, to theatrical parents. His mother, Julie Wilson (1924–2015), was an American singer and actress, "widely regarded as the queen of cabaret." His father, Michael McAloney (1924–2000), was an Irish actor and producer best known for his Tony Award-winning production of Brendan Behan’s Borstal Boy, an autobiographical play about a young member of the Irish Republican Army, which was the first Irish production to win top honors on Broadway.\n', '
Because his father wanted an Irish education for his two sons, Holt and his younger brother were sent to live with another family in Dublin, while his parents stayed in New York City, working. In Ireland, he attended national school in Howth. However, following his parents\' divorce the children moved back to the United States and Holt attended school in New Jersey. He was later sent to live with his maternal grandparents in Omaha, Nebraska, where he had a troubled childhood and was expelled from the Jesuit Creighton Preparatory School. At the age of 14, he ran away from home and took a Greyhound bus to Los Angeles to pursue his dream of becoming an actor but ended up with a job in a screwdriver factory unloading trucks. His parents eventually tracked him down and sent him back to Ireland to a boarding school in County Kildare that his father had attended forty years earlier.\n', '
He soon left Ireland and eventually graduated from Creighton Preparatory School. After high school, Holt went to France for college, first to study French at the Sorbonne and Art at the Paris American Academy, and later Theater at L\'École Marcel Marceau and L\'École Jacques Lecoq, but was expelled. Holt spent a summer studying Shakespeare in Oxford, and went with a production of Twelfth Night to the Edinburgh Fringe Festival in Scotland, before moving to New York City to begin his professional acting career.\n', '