McAllister, who spent time on loan with Shrews last season, got on the end of a Marcus Williams cross to volley past Chris Weale from the edge of the six-yard box.
Shrewsbury had several chances to equalise in the first half. Marvin Morgan failed to direct a header on target and Mark Howard saved Paul Parry's 20-yard effort, while Parry also fired narrowly wide following a free-kick.
Mark Wright threatened from close range but was forced wide and failed to get in a shot.
United's Nick Blackman fired beyond the far post after breaking into the area and then tested Weale with a shot which was beaten away by the Town 'keeper.
United's Tony McMahon and Shaun Miller both had efforts saved by Weale in the second half. At the opposite end, Howard kept out shots from Terry Gornell and Parry while a Wright effort was deflected wide.