Robert Marvin Hull, Jr. was born on January 3, 1939 in Point Anne, Ontario, Canada, where his father worked in a cement plant. The eldest son in a family of eleven children, Hull received a pair of ice skates as a Christmas present when he was three years old and took to the ice immediately. During his childhood he often cleared the ice so that he could skate on the frozen surface of the Bay of Quinte near his family's home. Before he had reached his teens Hull was playing alongside his father in a local amateur hockey league. His skills were so impressive that Bob Wilson, the head of scouting for the NHL's Chicago Blackhawks, signed the twelve-year-old to a contract committing him to the team. After playing for a juvenile-league team in Hesperer, Ontario in 1952, Hull joined the Blackhawks' junior-league affiliate in Woodstock, Ontario. He then spent the 1954-55 season playing for the Galt Black Hawks in the Ontario Hockey Association and the following two seasons with the St. Catherines Teepees in Ontario. He almost left the team after his coach, Rudy Pilous, told him to play left wing instead of center, which Hull perceived to be a demotion. After talking it over with his father, he decided to rejoin the team after missing four games over his protest.
At five feet, ten inches tall and weighing about 190 pounds, Hull was already an impressive figure on the ice as a teenager. Although he sported huge forearms and thighs and a barrel chest, Hull seemed to move faster on the ice than any of his opponents. His talent was on full display in 1956 when he was unexpectedly called to an exhibition game staged by the Blackhawks against the New York Islanders in St. Catherines. The high schooler scored two goals against the Islanders in the game. Desperate for new talent—the team had finished last in the league for the past four seasons—the Blackhawks brought Hull to the NHL in 1957 and named Pilous head coach as well.