Joseph Michael Medwick was born on November 24, 1911, in Carteret, New Jersey. He was the 4th child of Joesph and Elizabeth Medwick. He was of Hungarian decent. His parents immigrated to the US in 1893. His obituary stated that he was a four-sport star in high school, track, football, basketball, and baseball. He won all-state honors as a half-back in football and had many offers from colleges to play football. But he preferred baseball and was signed off of the sandlots by the St. Louis Cardinals.
He was 18 when he made his professional debut at Scottsdale, Pennsylvania.