Ferguson's return to the London Road club he left in the Championship's bottom spot in early November failed to generate the expected pulsating clash with play from either side slow and laboured and an atmosphere throughout that was rarely anything more than flat.
After Michael Hart had a shot scrambled away in the first minute, Preston spent the next half-hour on the receiving end but survived thanks to some inept finishing and a couple of impressive saves from Andy Lonergan.
The Preston goalkeeper went full length to his right to hold a header from transfer-listed Aaron McLean, who was making his first start of the year. The highly-rated Lonergan turned away a 20-yard drive from Josh Simpson before Chris Brown managed to get his body in the way of Charlie Lee's snap-shot from the corner that followed.
Simpson and Chris Whelpdale, twice, fired over the bar before Mellor broke the deadlock with a left-footed 34th-minute effort that finished up in the top corner of the Posh net, Ross Wallace getting the ball through after intercepting a wayward clearance from Kerrea Gilbert.
As the second period opened, Posh's Joe Lewis was the keeper under pressure, starting with a superb reflex save to tip over Richard Chaplow's close-range header.
Lewis held on to a shot from one-time Posh defender Sean St Ledger and Mellor went close to grabbing a second goal with one effort punched out by Lewis before a second shot was cleared off the goal-line by Ryan Bennett.
Posh rarely pushed forward with any real conviction apart from a late break by George Boyd whose low 25-yarder had Lonergan at full stretch as he held the ball at the second attempt.