A brilliant second-half showing saw East Coast storm home to end St Pats' season 9.12 (66) to 4.11 (35).
Having fallen victim to the Saints at St Helens the last time they met, it looked like the Swans would suffer the same fate early despite the home-ground advantage.
Trailing by 10 points at the main break, the Swans held St Pats to just six behinds in the second term to secure the victory, kicking seven unanswered majors in the process.
St Pats came out of the blocks strong, kicking 2.4 to their opponents' three points in the first quarter.
A pair of goals apiece in the second created an interesting contest before the home side blew St Pats out of the water in the first finals game in St Helens in 20 years.
Prominent goalkicker Nick Child led the way by kicking three, showing he hasn't missed a beat despite only playing eight matches this year after booting 264 goals in the past three seasons.
Teammate Shaun Cannon kicked two and was adjudged the Swans' best, joined by Adam Marland, Will Stoltenberg, Chae Evans, Cameron Upton and Majak Miar in gaining recognition.
At the other end, it was Jake King who booted three for St Pats, bringing his season tally to 65, finishing two goals short of Sonny Whiting's tally before he adds to it next week.
King was also named the best in defeat with Chris Hay, Luke Walsh, Alex Russell, Pat Rodman and Ryan Choveaux toiling hard.
The East Coast win means the Swans will face the loser of Sunday's Invermay Park match-up between Old Scotch and Old Launcestonians.
The Thistles have gotten the better of their opponents both times they've met this season, making them favourites for the contest.
East Coast's victory also meant the Swans wouldn't leave their home turf winless, after their reserves tasted defeat earlier in the day at the hands of Old Scotch.
Scotch came away 12.2 (74) to 6.7 (43) victors courtesy of a dominant second-half performance and James Bayles' four goals, which earned him best-on-ground honours in the victory.
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