Johnson opened the scoring with his 24th minute gem and Lewis Grabban netted the second in the 62nd minute of the East Anglian derby at Carrow Road to secure a sixth successive win for Norwich as they took full advantage of the defeats suffered by leaders Derby and second placed Middlesbrough on Saturday.
The Canaries appear well placed to make an immediate return to the Premier League after last season's relegation, with new boss Alex Neil having inspired a fine run of form since taking over in January.
Ipswich have slipped down to sixth and are now two points behind Norwich.