The second half also started quietly before Grimsby took the lead on 73 minutes after the linesman put his flag up most of the Swindon players stopped, but he then put it back down and Grimsby scored from close range with a goal from Toner.
Michael Timlin was sent off on 82 minutes for a second yellow card after a late foul reducing Swindon to 10 men for the final 10 minutes.
Swindon had a few late corners which goal keeper Phil Smith came up for, but Swindon were unable to do anything with them.
Overall a fairly poor and eventful game which leaves Swindon in 5th place. Their lowest place all season. Next Sunday Swindon are away to Lincoln City in a game which is also being shown live on Sky TV.