Geelong traded pick No.14 in this year's draft for the disenchanted 30-year-old and a third-round selection in 2028 on Thursday afternoon.
New Zealand-born Marshall had requested a move to the Cats for a second straight year after St Kilda recruited Tom De Koning to become their first-choice ruckman.
Cats football boss Andrew Mackie on Monday indicated an unwillingness to part with their first-round pick for Marshall after the Saints asked for a player to be attached to the trade.
St Kilda football boss Lenny Hayes hailed the fair return for Marshall, who has played 177 AFL games since being a rookie pick at age 21.
"We are pleased to have achieved (adequate compensation) by bringing in another first round selection, alongside our existing pick at eight," Hayes said.
Geelong needed to bolster their ruck stocks, with Rhys Stanley retiring and 21-year-old Mitchell Edwards still finding his feet at the level.
The Cats are among five clubs eyeing Nick Blakey as the Swans defender tests the water for a possible trade.
The 2024 All-Australian was one of five players who served a club-imposed ban through the AFL finals for breaching the club's behavioural standards at the Pullman Hotel in East Melbourne.
The 26-year-old has since been linked to a move despite being signed to Sydney until 2031.
Swans list boss Chris Keane said the club had not been told Blakey was meeting with rival clubs this week, but remained in talks with his manager Robbie D'Orazio.
"As we sit here right now, these are all hypotheticals," he told Trade Radio.
"It's certainly not something we've actively pursued, and we feel confident that he could walk back into our footy club.
"But given the calibre of player that he is, and the fact that he is contracted for five more years, we feel like making any deal happen would be significantly challenging."
Blakey's preferred option is reportedly Geelong, but the Cats have limited draft capital after executing trades for Marshall and Essendon's Jordan Ridley.
North Melbourne have expressed interest for Blakey, whose father John played in the Kangaroos' 1996 and 1999 premierships across 224 club games.
Essendon are another suitor, with Blakey's former mentors Mark McVeigh and John Longmire joining the club and the Bombers boasting salary cap room following the exits of Zach Merrett and Ridley.
"We haven't had any indication off Nick's management that he's looking to leave Sydney," Essendon list boss Matt Rosa told Trade Radio.
"But certainly we've had some discussions internally and we'll continue to do that."
Blakey has reportedly already met with Gold Coast coach Damien Hardwick, while St Kilda are also expected to make contact.
Any trade for Blakey would likely need to be approved by a club board, given the ongoing police investigation into an alleged sexual assault at the hotel.
Swans vice-captain Isaac Heeney, star midfielder Chad Warner, tagger James Jordon and Blakey were interviewed by police but no charges have been laid.
Riley Bice, another Swan banned for round 24 and the finals campaign, is not being investigated.
In other developments, young Hawk Aidan Schubert has agreed to be traded to Gold Coast in a separate deal that is expected to allow Ben King join Hawthorn as a free agent.
The Suns' 72-hour window to match the Hawks' offer for King or accept compensation, likely pick No.7 in the draft, ends at 4.15pm (AEDT) on Friday.
The Western Bulldogs are yet to file free agency paperwork for Port Adelaide's Zak Butters ahead of Friday afternoon's deadline.
Port can accept draft compensation, which would currently deliver pick No.5, or match the Bulldogs' offer and force a trade.
The trade period closes on Wednesday evening.
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