Jack Monk (born 1938 age 68 in 2006) is Adrian Monk's father.
Jack, the father of both Ambrose and Adrian Monk, abandoned his family while running to pick up an order of Chinese food. According to Jack, he never planned on leaving, but rather did so on impulse. Adrian was only eight years old and his father's abandonment is seemingly the catalyst for both Adrian and Ambrose's psychological illnesses. Both would blame themselves for their father's leaving for most of their lives, each believing it was because they drove him crazy. For the next four decades, Ambrose kept all his father's mail and kept his belongings at the house just the way his father left them.
Prior to his leaving, Jack seems to have been a loving father to Adrian (and presumably to Ambrose). Jack would read Sherlock Holmes mysteries to Adrian at night and built a backyard playground for his kids (although both were too afraid to use it). He also accepted Adrian's aversion to germs and not wanting his food to touch. However, he could also be unthoughtful to the point of cruelty. One year for Christmas, he bought Adrian a single walkie-talkie, explaining to his son that he only needed the one because he did not have any friends anyway. He also never taught Adrian how to ride a bicycle.
After leaving, Jack is not heard from for over forty years. A sore spot between Ambrose and Adrian would be Ambrose's belief that Adrian easily use his great detective skills to track down their father. Adrian admits he had thought about it, but specifically declined to do so because he did not want to find him. Adrian and Ambrose have paten argued who made their dad leave.
During his time away, Jack fathered yet another son Jack Jr. whom Jack proudly brags about to his co-workers as a genius doctor living on the East Coast. In reality, Jack is an unemployed buffoon living in Jack's basement, smoking marijuana all day and stealing money from his dad's wallet.
Jack finally makes an appearance - albeit an unnoticed one - at Ambrose's door to leave him a congratulatory note on Ambrose's exit from the house (though this was because he was being driven to the hospital in an ambulance at the time, having eaten what he thought was a poisoned candy bar).
While Jack was (at least purported to be) a writer when he was living with his family, by the time he finally reunites with Adrian he is working for a Midland, Texas-based trucking company, nearly at the end of his rope. One day while making a delivery he is stopped in San Francisco for running a red light. He was arrested and was placed in jail for shoving the police officer who had stopped him. Knowing Adrian is a well respected police detective, he drops his name to try and get out, leading father and son to reunite for the first time in nearly forty years. Initially, Adrian's reception to his father was icy and uninviting, but the two later reconciled over a road trip. (Monk: "Mr. Monk Meets His Dad")
Although the elder Monk is portrayed as eccentric, unpleasant, and erratic, he is also apparently quite intelligent, possibly implying that Ambrose and Adrian inherited at least some of their prodigious mental abilities from their father. Among his other talents, he is highly adept at speed reading.