Ryan Taylor came back into the starting line up and he came up with the winning goal to give the Millers a much needed boost.
They always looked the better team and for a change keeper Andy Warrington was rarely called upon as he enjoyed one of his easiest afternoons of the season.
An early Peter Holmes free kick brought no reward before Tom Cahill did well to get in a cross that went behind.
Then these two linked up well but the cross from Cahill on the turn was too deep and it wasn't until the 11th minute that town launched an attack that saw Till's low cross find Clark whose close range shot was blocked.
Back came United again for Jamie Green to cross and Cahill's shot on the turn was saved by the keeper before a 30 yarder from Taylor went only just wide.
A visiting corner brought no reward while at the other end a free kick was caught easily by Warrington but the game saw too many misplaced passes from both teams.
Graham Coughlan had to head out a cross from the left for a corner before Cahill had another effort blocked but it was the Millers who broke the deadlock on 40 minutes.
Mark Hudson hitalong ball over the defence and as Bennett hesitated, Taylor nipped in to win the ball and went on to score with a fine shot from a narrow angle with the ball going in off the near post.
Another Rotherham corner was dealt with and at the start of the second half a good move saw Cahill link well with Holmes only for his shot to be saved..
A Marc Joseph free kick brought no reward while Town replied with Till crossing but the danger was snuffed out again.
Then Town's Taylor went on a good run but his shot was blocked before Holmes had a shot saved after another good move. Three home corners were dealt with efficiently and Holmes was in the action again on 70 minutes having another effort kept out.
Substitute Marc Newsham had a good shot saved while at the other end former Rotherham striker Martin Butler hit an effort just over the bar.
But whatever Town tried to throw at the Millers they were repelled with ease and United always looked in command and they went on to safely bag another well deserved three points.
Grimsby: Barnes, Clarke (Bird 78), Newey, Bennett, Fenton, Till, Toner (Taylor 46), Boshell, Hegarty, Bore (Jones 68), Butler. Subs: Montgomery, Normington.
Rotherham: Joseph, Mills, Coughlan, Tonge, Harrison, Hudson, P. Holmes, Green, Taylor, Cahill (Newsham 67). Subs: Oxley, Haggerty, D. Holmes, Yates.
Referee: Andy Penn (West Midlands).
Attendance 3,583 (558 away).
