Goals from Carey (39) and Jervis (71) gave the visitors a deserved three points with Northampton's bad day compounded by the late dismissal of Josh Lelan for a second yellow card.
Two successive defeats have put the brakes on an encouraging start to Town's season but they do now have a big League Cup tie at Newcastle United to look forward to in midweek although that will do little to lift Chris Wilder's spirits.
He admitted afterwards that Plymouth had been the better side over the 90 minutes and felt the visitors had it 'easy' in the second half due to the lack of quality on display from the Cobblers.