Mellor, who was deployed as a winger, netted the decisive goal to give North End their first away win since October.
The former Liverpool man drifted inside from the right, engineered himself some room on the edge of the box before unleashing an excellent left-footed strike into the across goalkeeper Joe Lewis and into the right hand corner who could do nothing but pick the ball out of his net.
After his impressive display against Chelsea, in what was his first start since November, Michael Hart kept his place in the side allowing Billy Jones to take up midfield duties.
The former Aberdeen man put in another solid performance from which he can build from and cement his place in Ferguson's team.
As well as it being North End's first win at London Road, Tuesday's game will also be remembered as the first time Preston supporters got a glimpse of Danny Welbeck in action for the Lilywhites.
The on-loan Manchester United striker had a solid debut in which he could have got himself on the score sheet. Instead of striking first time from a Ross Wallace cut-back, Welbeck took a touch allowing the oncoming defender chance to block his shot.
Additionally, it was also Darren Ferguson's first win as Preston manager pictured applauding the traveling support.
For more pictures from Tuesday's win over Peterborough or indeed any other game, click here.