Two minutes earlier, Luton had been pegged back to 3-2 up on the night, which meant that, at 4-4 on aggregate, Ipswich would have reached the third round on away goals. But Johnson joined a 10-man Luton attack in the closing seconds - and headed his first goal of the season to complete a night of high drama at Kenilworth Road.
Ipswich, 2-1 up from the first leg, took a first-half lead through David Johnson. A fine individual goal by Andy Fotiadis levelled the score on the night and then Stuart Douglas struck to earn extra time.
Luton's captain, Steve Davis, headed home to put Luton 4-3 up on aggregate but then, trying to clear a cross by Danny Sonner, he headed into his own net. His despair was forgotten, though, when Johnson headed the winner from a corner.
Bournemouth's experienced striker Mark Stein scored twice to knock out Wolves at Molineux. Despite Mark McGhee's team dominating the second leg, they bowed out 3-2 over the two legs. Robbie Keane scored their only goal.