It had looked like the visitors were going to pick up their first victory since August 17 when Tom Bradshaw netted his third goal of the campaign on the hour mark.
But Town kept going and eventually earned a point with four minutes remaining, thanks to McManaman's first strike since netting for Wigan in November 2018.
Harry Cornick could not convert an inviting cross at the far post, while James Collins' first-time effort from Brown's clever corner was excellently repelled by Lions keeper Bartosz Bialkowski.
The hosts had another opening in the 34th minute as Pelly-Ruddock Mpanzu fired over the top from 22 yards.
Ryan Tunnicliffe might have done better with an opening against his former club, seeing his effort deflected across goal and away.
Half-time substitute Ben Thompson put an ambitious header wide for the visitors early in the second period as the game started to open up.
Bialkowski made a superb save to keep the score goalless, somehow finger-tipping Collins' volley from a corner on to the bar.
The visitors' efforts were mainly hopeful attempts, with on-loan Brighton midfielder Jayson Molumby putting a volley from range into the away end.
However, out of nowhere, the Lions led on the hour mark as they broke into the box and although Simon Sluga did well to save Jed Wallace's angled drive, he was powerless to prevent Bradshaw sliding home the rebound.
Collins sliced over from range, before Matt Smith missed a glorious opportunity to double the lead, heading badly wide from close range, with that miss to prove crucial.
Brown curled off target from the edge of the box, while Bialkowski produced further heroics to deny Collins from a matter of yards after Kazenga LuaLua's low delivery.
Sluga beat away Thompson's effort from range, before the hosts had a deserved share of the spoils.
Substitutes combined as Luke Bolton reached the byline and his cross was clinically side-footed home by McManaman.
Molumby's dipping volley was only narrowly over with Sluga at full stretch late on as the Lions almost nabbed a winner.