The Lions have sent the 20-year-old ruckman and pick No.48 to the Saints in exchange for picks No.25 and 41.
Longer, who played eight games in his two years at the Lions, was determined to return home to Victoria when it became clear that Matthew Leuenberger had a hold on the main rucking berth.
"I'm very relieved," Longer told Gillette AFL Trade Radio.
"It's been a stressful few weeks for everyone involved. I'm elated with what the result is. I'm pretty excited to move on and get stuck into it with St Kilda."
He was initially keen to get to Hawthorn until the 2013 premier moved swiftly to trade for St Kilda's Ben McEvoy.
"To be very blunt I thought it was going to get done," Longer said.
"But the way it has panned out I couldn't be happier with where I am."
Longer will now compete with Tom Hickey for the No.1 ruck berth at the Saints, who are likely to bring in the experienced Darren Jolly as a delisted free agent next week to provide support and mentoring for their emerging big men.
"It's a big thing with McEvoy leaving but there's still three developing ruckman, me included, so I think it's a good position to be in because there's three of us fighting to make positions," Longer said.
"It's worked out pretty well for us."
The Lions have on-traded their third round selection in the NAB AFL draft for Geelong ruckman Trent West.
The Lions are also expected to trade for Geelong ruckman Trent West before the trade period closes at 2pm on Friday.
In the past 24 hours they have traded Sam Docherty to Carlton, Elliot Yeo to West Coast, Jared Polec to Port Adelaide and Patrick Karnezis to Collingwood.
