Rendell steered the Blues ahead but Sam Higgins smashed in a free-kick to equalise for Chelmsford just before the hour.
But 24-year-old Rendell bagged his eighth of the season to put the League Two side back ahead and Stuart Beavon clinched victory with six minutes left.
The scoring began when Matt Bloomfield's corner was met by Gareth Ainsworth's volley and Rendell reacted quickest to turn the ball in.
The Essex outfit punished their hosts in the 59th minute when Higgins fired in a free-kick from the edge of the area.
Anthony Cooke curled wide for City before the Blues countered and Kevin Betsy's cross found the unmarked Rendell lurking at the far post to score. Beavon grabbed another on the counter to secure victory and condemn Chelmsford to their 10th straight defeat against league sides.
Source: PA
Source: PA