Mensah caught the eye with a number of powerful challenges and interceptions as he made his first Black Cats start against Birmingham on Tuesday night.
The Ghanaian centre-half is expected to retain his place in the starting line-up for tomorrows Premier League game with Wolves, and Campbell claims his midweek display did not come as a surprise to any of his Sunderland team-mates.
Mensah has been winning tackle after tackle on the training ground, and the Black Cats forwards have already learned to avoid him wherever possible.
Players bounce off John Mensah whenever they go up against him in training, said Campbell. Hes a bit of a solid bloke isnt he
He has been absolute quality since he arrived, and hell be competing to start now.
Hes not afraid to put his foot in, and hes exactly the same in training as he is in a match.
Mensah was not the only player to impress in Tuesdays 2-0 win over the Blues, as Campbell also made a positive impression in only his fourth Sunderland start.
The 22-year-old scored his first goal since a summer move from Manchester United, and with Kenwyne Jones place in the starting side having been called into question since Steve Bruce arrived to replace Ricky Sbragia in the summer, Campbell could be at the start of an extended run in the team.
There was a little bit of me that thought I would be playing back up to Kenwyne and Darren (Bent), he said. But in football, things change.
Theres only two of those guys and I knew I would get my chance. Its all about me taking the chance when I get it, and Id like to think I have.
Ive scored, but I dont know whether I will be in on Sunday or not, no one does really.
I just have to keep focused and training hard.
Tomorrows game is Sunderlands first Premier League outing since last weekends disappointing defeat at Burnley.
Campbell admits some harsh words were exchanged in the wake of the Turf Moor setback, and claims Bruce is determined to change the mindset.
Ive not been here too long, but the players say the mood around the place is changing, and you can see it among the fans. They are sensing that things are on the up.