Thornbury now sit six points above the drop zone but Chipping Sodbury Town have two games in hand and could yet pull Thornbury back into the mix.
It had been a positive start to Easter Monday’s trip to Wantage, a team who have taken some scalps at home in the 2022/23 campaign.
Harry Kenneth-Brock and Stanley Wyatt scored either side of Luke Campbell’s equaliser to give the visitors a 2-1 lead at half-time.
But three goals in five second-half minutes after the break saw The Freds turn things around as Rhys Jay Wells, Joshua Davis and Campbell made it 4-2.
Thornbury had been stunned but pulled one back in the 81st minute through Kenneth-Brock to give them hope of salvaging a point.
But substitute Thomas Wynn-Davis put the result beyond doubt before Davis converted a penalty to add icing on the cake and a 6-3 socreline.
Thornbury have just one game left this season and must win it to ensure they will survive the drop.