Monahan was selected first overall by the Montreal Canadiens in the 1963 NHL Amateur Draft of 16-year-old players—the first pick of the NHL's first draft. The next season, he played junior B hockey with the St. Michael's Buzzers in Toronto before moving up to the junior A Peterborough Petes in the Ontario Hockey Association, where he played from 1964 to 1967. In his final junior year, he turned into a top scorer playing on a line with Mickey Redmond, the league's leading goal scorer. Monahan scored 30 goals and 84 points in 47 games on what was otherwise a weak Petes team.