Coventry are out of the FA Cup after being beaten 2-1 by Portsmouth in the third round.
Leon Best put Coventry ahead in the first half, but an own goal by captain Stephen Wright in the final minute of the game gave Pompey another chance.
The visitors made the most of their opportunity in extra time, with Mokoena heading home from a corner in the closing seconds of the match.
The result means Portsmouth now face Sunderland in the next round.
Elsewhere, Tuesday's other FA Cup ties saw Birmingham beat Nottingham Forest 1-0, while Cardiff held Bristol City to a 1-1 draw.