The 24-year-old midfielder would have been eligible to test his value using free agency at the end of next season, having been drafted in 2006.
But after beginning to establish himself at senior level in 2011 and taking another step forward last season, he has been rewarded with a deal that will see him remain a Saint until at least the end of 2016.
"I just always wanted to be a one-club player. I've stuck by the Saints and they've stuck by me," Armitage told AFL.com.au.
The hard-bodied midfielder has played 71 games, including 43 in the past two seasons.
He admitted that he contemplated leaving St Kilda after struggling for senior opportunities in his early years, but his desire to be a one-club player and stay with teammates he had built relationships with swayed him.
"A couple of years ago, some people wondered why I didn't leave, but you get to a club and you just form really strong friendships and tight bonds with certain players," he said.
"When I wasn't getting a game you do consider [leaving] but I spoke to certain people and they said eventually I was going to play and I had to keep chipping away.
"I wasn't going to push Lenny [Hayes] or 'Dal' [Nick Dal Santo] or Joey [Leigh Montagna] or someone like that at that stage, especially in the finals of '09 and '10.
"I just had to bide my time and finally it's starting to pay off."
Armitage was promoted to the leadership group this season and credited second-year coach Scott Watters for giving him the confidence to believe he belonged at senior level.
"Just at training, if I made a mistake I was ready for a spray and he just pulled me aside – and he did that with a lot of blokes, Sean Dempster, Jimmy Gwilt – and said, 'I'm not going get up you if you make a mistake, as long as you make a mistake and try to do something good'," Armitage said.
"I started to back myself in a bit more and hit different kicks and it started paying off.
"He was enormous when he came in and just gave me that confidence, because you need confidence as a player otherwise you're going to drop your head and it eats away at you."
Watters said he believed Armitage would improve further after a pre-season that had left him "in terrific nick".
"Sometimes as a coach you have to make sure the player knows you have a vision for them and that you're prepared to walk alongside them to get them to that point," Watters said.
"I believe in him as a player, but ultimately he's been responsible for forging forward, so he should take most of the credit for the work he's put in to get where he's at.
"I certainly have a lot of faith that he can still get better as well."
David Armitage is a midfielder in NAB AFL Fantasy. He averaged 89.19 points in 2012. Register your team at our AFL Fantasy Hub.
Jennifer Phelan is a reporter for AFL Media. Follow her on Twitter @AFL_JenPhelan.
