Barton was guilty of a vicious stamp on Mendes' ankle late in the first half, which left the Portuguese international rolling around in agony on the halfway line.
Amazingly referee Mike Dean allowed Barton, who won his first cap in Wednesday's friendly debacle with Spain, to escape with only a yellow card.
Pompey manager Harry Redknapp was so furious with the sickening challenge he was sent to the stands after storming into Dean's dressing room at half-time to vent his anger.
The incident came five months after Mendes was left hospitalised following Ben Thatcher's brutal elbow attack during Pompey's 0-0 stalemate at the City of Manchester Stadium.
Barton's pre-meditated assault came after Mendes had fired Pompey in front with a stunning volley in only the fourth minute.
Matt Taylor's right-wing corner was punched away by goalkeeper Andreas Isaksson and the ball broke for Mendes on the edge of the penalty area.
The ex-Tottenham midfielder brought the ball down with his right foot before picking out the top right-hand corner of Isaksson's net with a superb left-foot piledriver.
Lomana LuaLua had Isaksson scrambling his goal six minutes later when he cut in from the left and drilled in a powerful right-foot shot which was arrowing towards the bottom left corner before the Swede somehow tipped it around the post.
The busy Isaksson was called into action once more in the 13th minute when Taylor came bursting into the penalty area to latch on to Gary O'Neil's through ball and his shot was turned over for a corner.
Glen Johnson came within inches of his first Pompey goal six minutes later with a cross-cum-shot that evaded Isaksson and sailed just wide of the right-hand upright.
Isaksson was almost left red-faced when he flapped at LuaLua's ambitious shot across the face of his goal but managed to redeem himself by punching the ball behind for a corner.
Bernardo Corradi almost nicked City a goal they scarcely deserved ten minutes before half-time when he hammered a right-foot volley into David James' side-netting after Darius Vassell had headed the ball into his path.
Redknapp was forced to watch the second half from the directors' box and leave technical director Avram Grant to run things from the sidelines.
His misery was compounded when Barton helped set up City's equaliser 17 minutes into the second half.
Barton played in Vassell down the right and he crossed for strike partner Corradi to rise above Sol Campbell and head into the right-hand corner from six yards.
Isaksson prevented Pompey from restoring their lead soon after when he pulled off a stunning save to keep out substitute Lauren's close-range effort after Taylor had nodded O'Neil's centre across the face of goal.
Justice was done nine minutes from time when top-scorer Nwankwo Kanu fired Pompey back in front. Johnson threaded through a pin-point pass and Kanu slid the ball past a despairing Isaksson.