The Blues have been bracing for the ruck's exit for the whole year due to the lucrative offer from the Saints.
Five days after Carlton's season ended, De Koning informed the club in his exit meeting that he is leaving to take up a monster contract with the Saints.
Although he is a restricted free agent, the Blues won't be matching St Kilda's offer for a player they drafted with pick 30 in 2017.
"To be really honest, this has been the toughest decision I have ever had to make, leaving a club I've loved being part of over the last eight years, with friendships I will take with me for the rest of my life," De Koning said.
"In the end I have made a decision that is best for me, but I just really want to take this opportunity to thank the entire Carlton community who has embraced me since I was drafted – in particular the cheer squad, who have sponsored me and been an amazing support."
St Kilda football boss David Misson said the club was "very pleased" De Koning has chosen the Saints.
After his 100th and final game for Carlton, the 26-year-old was publicly acknowledged by Blues president Robert Priestley in the rooms in front of players, coaches and supporters.
De Koning even spoke after Priestley in what amounted to a farewell speech, telling his Carlton teammates: "Keep going and do good things soon, so all the best.
"Thanks for all the good times over the past eight years."
Asked about De Koning's future post-game, Carlton coach Michael Voss was resigned to losing him.
"When you wait this long, we're not expecting the best of news," Voss said.
"One thing I've learned about this game is that people come and go, but the club keeps going."
Carlton list boss Nick Austin wanted to keep De Koning, but the club wasn't in a position to financially compete with the Saints.
"Ultimately the best interests of the football club come first and remains at the centre of every decision we make in regards to our list management," he said.
"Tom and his management have been nothing but professional throughout this process, and while it is disappointing to see Tom depart, we will now look to maximise the flexibility that now exists within our playing list as we move into the off-season period."
St Kilda's bold plan to secure players with big money appears to be paying off after recently retaining star Nasiah Wanganeen-Milera for a further two years.
Wanganeen-Milera was heavily pursued by Adelaide and Port Adelaide, but the 22-year-old decided against moving back to South Australia.
St Kilda are also favourites to win the services of West Coast premiership forward Liam Ryan, who on Tuesday told the Eagles he wanted to leave the club despite having a year to go on his contract.
Carlton will also lose third-generation Blue Jack Silvagni, who is weighing up options from Collingwood, the Western Bulldogs and St Kilda.
Silvagni's father Stephen works in list management at the Saints.
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