Bailey Gooda played his first 45 minutes of the pre-season, former Tadcaster defender Kieran Dyer battled throughout, and Dylan Cogill and Simon Heslop both played well, but otherwise it was a hugely disappointing performance from Boro.
The opening half-hour was largely uneventful, with Heslop having a shot blocked by ex-Boro full-back Jack Johnson, and Luca Colville fired the rebound wide, before the game exploded into action and it was Tadcaster who created the chances.
Midfield dynamo Sam Barker teed up Charlie Jebson-King, who fired inches wide, then another lay-off from Barker sent Brett Agnew through on goal, but the Boro keeper saved at his feet.
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They went ahead on 35 minutes when a Barker shot was saved, Agnew fired in the rebound but Thornton blocked, and the ball broke to Nathan Dyer who slammed his shot into the bottom left corner.
In further Tadcaster chances, livewire Barker shot over and a free kick into the box from Josh Grant was hooked over the bar by Dec McGivern.
Boro looked disjointed, but Kieran Glynn passed to Colville, who shot narrowly over, then early in the second half a ball forward from Colville was chested down by Glynn towards Jake Charles, who fired inches over.
Kieran Burton went on a couple of surging runs forward, and a cross from James Cadman was headed at the keeper by Ryan Watson as Boro tried to fight back, but it was Tadcaster who added a crucial second goal.
A shot from Jebson-King was cleared off the line by Dylan Cogill, but on 72 minutes referee George Roberts penalised Kieran Glynn for a foul on Barker, and Harry Jessop sent the keeper the wrong way from the spot.
Colville blazed over, then Michael Coulson laid off to Cogill, whose 20-yard shot was saved.
Colville sprayed out a terrific ball to Dom Tear on the left, and he cut inside and passed back to Colville, whose shot was saved. It was all Boro in the last 15 minutes, but they never tested the keeper.
Coulson and Watson both fired over, and a long-throw from Burton was flicked on by Cogill to Bailey Gooda, whose attempted shot was blocked as Tadcaster held on for a well-deserved win.
Boro: Trialist A, Weledji (Cadman 59), Burton, Heslop (Watson 59), Thornton (Gooda 46), Cogill, Plant (Maloney 59), Colville, Charles (Coulson 59), Glynn, Trialist B (Tear 59)
Man of the Match: Kieran Burton.