A goal after less than two minutes cost slow starting Ayr a chance of Premier Sports Cup redemption.
Graham Carey's 20 yard volley caught United cold in Perth.
But Lee Bullen will have been somewhat encouraged by what he saw from his shadow side, as they refused to buckle ahead of next week's league curtain raiser against Arbroath.
Bullen rung the changes with little riding on this final group stage match and introduced six fresh faces from Tuesday visit to Annan.
Finn Ecrepont was handed a rare start at left back in place of the suspended Paddy Reading, while Daire O'Connor and Jayden Mitchell-Lawson both started in an attacking 4-4-2.
But the game was barely two minutes old and United had fallen behind with slack defending proving their downfall.
A cross from the right was only half cleared by Alex Kirk and Carey lashed the loose ball past Aidan McAdams' despairing dive from 20 yards.
The early goal threatened to set a worrying tone but Ayr rode a flurry of corner kicks as Saints looked to put the game to bed in the opening stages.
McAdams' goal remained relatively untested despite Saints looking dangerous although Carey curled a free kick just wide on the half hour before Jamie Murphy had a shot deflected wide when Saints should have done better.
That sparked Ayr into life and Sam Ashford should have levelled just before half time when he shot against Elliott Parish with only the keeper to beat from eight yards.
United started the second half brightly and Dipo Akinyemi had a header saved from a Paul Smith corner as they threatened.
O'Connor then sent two efforts over the top as Ayr continued to press but they couldn't make the most of their possession.
Saints sub Taylor Steven tested McAdams with a low drive on the counter attack but it was a brief threat from Saints, who did little after the break to suggest they would extend their lead.
And Ayr missed their big chance to level matters late on when Ashford led a counter attack but with sub Mikey Hewitt unmarked and screaming for a pass, the striker went for goal but shot over the top.
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