On November 18, 2017, in a game against the Philadelphia Flyers, Monahan scored his first career hat trick in the second period to help the Flames win 5–4. However, his season was cut short in March due to injuries. During the following month, Monahan underwent four surgeries but was expected to be able to play during the 2018–19 season.
Monahan scored his 100th career goal against Andrei Vasilevskiy of the Tampa Bay Lightning on February 23, 2017. He is the 6th youngest active player to achieve this milestone, joining the elite company of Alexander Ovechkin, Sidney Crosby, Jaromír Jágr, Steven Stamkos, and Patrick Kane. His 100th goal also marked his 20th of the season, marking the 4 consecutive season he has scored at least 20 goals.
He became the fastest player in the Flames’ franchise to record 9 career overtime goals when he scored his 9th on December 7, 2017 vs the Montreal Canadiens in a 3-2 win.