Markworth, who suffered a ruptured anterior cruciate ligament during the pre-season, is yet to make his debut and was due to come out of contract at the end of this season.
A quick half-forward who can also play through the midfield, he was drafted from the Calder Cannons with pick No.35 at the 2011 draft and was an emergency on three occasions last season.
The 21-year-old can now focus on his recovery and aim to make an impact on field in 2014.
"It gives me a bit of security and I can concentrate on my rehab and getting back next year," Markworth told saints.com.au.
"They (St Kilda) have been really supportive. Ever since it happened they’ve said the security is always going to be there so once it’s down on paper it is very reassuring.
"It’s good now to concentrate on the rehab and hopefully play some good footy in 2014.”
Markworth, who suffered the injury during the Saints' high altitude training camp in Colorado in December, said he was pleased with his recovery so far.
"The knee is going really well so everything is on track," he said.
"I’m running around and getting fit now so once I’m out on the track it is all positive."
He said teammates Lenny Hayes, Sean Dempster and James Gwilt, who have all been through knee reconstructions, had been valuable sources of advice.
"I’ve had a chat to all three of them and there is not much you can say when it happens but they are always there for a chat when I need them," he said.
Markworth played 14 games for Sandringham in the VFL last season, averaging 13 disposals and kicking 10 goals.
Nathan Schmook is a reporter for AFL Media. Follow him on Twitter: @AFL_Nathan
