The 21-year-old put pen to paper to remain in red, white and black until at least the end of 2020.
Recruiting and List Manager Tony Elshaug said McCartin was an important signing as the club continued to develop its young talented list.
“It’s a great sign when one of your emerging players recommits to the club long term as Paddy has done,” Elshaug said.
“He has become a really popular member of our playing group and obviously sees a bright future at the Saints.
“Paddy has shown great character and resilience in the way he has confronted every challenge over the past three years.
“His work on the training track in the latter stages of last season and throughout the pre-season has been first rate and we’re excited by the player he can become in the coming years.”
McCartin said he was fully committed to helping St Kilda become a genuine contender.
“I love the club and I’m excited for what the future holds,” he said.
“The club has invested a lot in me and I look forward to repaying their faith.
“There is a great feeling among the group and we are really hungry to take the next step this season.”
McCartin returned to full training last week after suffering a minor foot sprain in early December.
He has played 22 games and kicked 22 goals since being drafted to the Saints with pick No. 1 in the 2014 AFL Draft.