The hosts raced into a two-goal lead after just 25 minutes thanks to Martin Smith's brace, but a Ciaran Toner penalty and a Ricky Ravenhill strike ensured the Mariners headed home with a point.
Darlington started brightly and visiting goalkeeper Phil Barnes had to push Craig James' 35-yard drive over the bar after just six minutes.
However, Barnes was beaten two minutes later when Smith fired home from 12 yards after the Town defence failed to clear their lines.
A rare Grimsby attack saw Ravenhill force Quakers goalkeeper Sam Russell into a fine 20th-minute save, but within five minutes the visitors had fallen further behind.
Michael Cummins collected a Kalusivikako Ngoma pass on the edge of the box and produced a deft touch to play in Smith, who strode into the area before firing past Barnes from 15 yards.
Grimsby responded strongly and Nick Fenton and Toner were both close to pulling a goal back, before Russell denied Peter Bore with a superb point-blank save shortly before half-time.
They kept up the pressure after the interval and Russell did well to deny Gary Croft before finally being beaten in the 66th-minute.
Darren Holloway fouled Tony Thorpe in the area to earn a red card and present Toner with a spot-kick, which he calmly despatched to pull Grimsby back into the game.
The hosts struggled to cope with their numerical disadvantage and it was no surprise when Ravenhill equalised eight minutes from time, smashing the ball home from close range after Russell had parried a Thorpe shot.