At Roker Park he was seen as a fine prospect and during Sunderland's first post-war season he vied for the 'keeper's shirt with Leslie Scott and James Dempster, making 19 league appearances.
In the 1921–22 season he kept 26 clean sheets as Southampton took the Division Three South title on goal average from Plymouth Argyle.
His slender build was frequently the targe of banter in the dressing room; players in the bath would cry out in mock alarm: "Look out, the plug's pulled – we don't want to lose Tom down the drain!
In the first Fourth Round match against West Ham at The Dell Allen was injured as he bravely thwarted Vic Watson and was replaced in the replays by veteran 'keeper Herbert Lock.
In the match played at Stamford Bridge on 28 March 1925, Saints' defender Tom Parker had a dreadful afternoon, first scoring an own-goal, then suffering a rare miss from the penalty spot (shooting straight at the 'keeper) before a mix-up between him and goalkeeper Allen gave Sheffield their second goal.
In the first match, at The Dell, Allen collided with his teammate Michael Keeping and had to leave the pitch with bruised ribs.