Jack is played by Scott Baio.
Jack was there only in seasons one and two (1993-1995), then he left to open his own practice in Colorado, where he finally became a rich.. He also could ski there.
He was Mark's resident before Jesse came, and was also like a son to Mark. He was loved to Amanda, but he though that Amanda didn't feel the same, and that was the reason why he left (The Silent Partner-book). But he came back to visit in The Silent Partner-book.
A womanizer whose rakishness and boyish charm coupled with his penchant for drop-kicks are often utilized by Mark Sloan, whom he idolizes, seeing him as somewhat of the father figure he never had growing up on the tough streets of Bensonhurst. With Mark's help he became a Doctor. He was helpful to Mark, Steve and Amanda, and he liked the action of crime. He thought it was nice to catch bad guys with Steve, and be the cool guy.
His godfather was the head of a crime gang, which inspired Jack to battle against crime. Jack was named One of the Top Bachelor's of the Year.