A second-half own-goal from Darren Murray saw Linfield beat Portadown after both sides were reduced to 10 men six minutes before half-time.
Portadown's Chris Ramsey and Linfield's Mark McAllister were red-carded after an altercation off the ball.
After David Miskelly saved from Niall Quinn in the first half, Quinn provided the corner to which Philip Lowry got the vital touch in the 47th minute.
Miskelly saved a Michael Carvill penalty with nine minutes remaining.
The Blues enjoyed the bulk of the possession throughout and McAllister had a shot blocked before his dismissal.
Soon after Murray's unfortunate own goal, Carvill tested Miskelly with a turn and shot.
The Ports goalkeeper made a fine save from the Linfield forward's penalty after Daryl Fordyce had been brought down inside the area by Brian Gartland.
The result sees the Premiership champions leapfrog the Ports into seventh position in the table.