Rotherham made five changes to Satruday's win over Mansfield including a debut for loan 'keeper Jamie Annerson, and started brightly. Ryan Cresswell, Jamie Yates and Marcus Bean all went close in the first 20 minutes, with Marc Newsham also having a shot tipped away by ex-Millers 'keeper Phil Barnes.
O'Grady broke the Grimsby resistance on 19 minutes when he lashed in from the edge of the box after combining with the lively Newsham, and a series of corners followed with the away side offering little other than a couple of flag kicks themselves.
However as we've come to expect in recent weeks the tables were turned after the interval as the Millers were forced onto the back foot. Kieran Toner drove a shot just over Annerson's bar and the young stopper did well to beat away Till's curling effort, before the latter levelled the scores on the hour.
Town broke clear from a Millers corner after Bean lost out in a 50-50 tussle and after combining with Toner with a smart give and go Till finished with aplomb in front of the 205 travelling supporters.
Rotherham: Annerson, Tonge, Cresswell, Sharps, Hurst, Peter Holmes (Brogan 69), Bean, Danny Harrison, Yates (Cahill 88), O'Grady (Derek Holmes 74), Newsham.
Subs Not Used: Warrington, Coughlan.
Grimsby: Barnes, Bennett, Fenton, Whittle, Newey, Till, Clarke (Jones 24), Bolland, Boshell, Toner, North (Taylor 46).
Subs Not Used: Montgomery, Bore, Jarman.
Att: 2,362 (away 205)