Saturday's win took England within touching distance of the World Cup quarter-finals.
Mark Cueto and Chris Ashton both scored hat-tricks in a 67-3 victory, with England securing the bonus point as early as the 31st minute.
Johnson had been unhappy with England's "sloppy" performance last time out against Georgia which led to some heated discussions among the camp about what had gone wrong.
But the boss had few complaints after Romania were comprehensively beaten at the Otago Stadium, despite his side conceding 12 penalties to Romania's 11.
He told ITV1: "These games can be awkward in a way and I thought we kept our discipline pretty well. Romania worked as hard as they could for 80 minutes and had a real crack at us. You have these games at the World Cup, we got off the field with no injuries so that's the main thing for us.
"Their priority (for Romania) may be Georgia in the week and winning that game but they certainly came and had a crack. It's one of those games where things maybe break for you differently than it would do in other games, you've just got to keep your discipline and play some decent stuff.
"It was job done tonight and we go into a massive week next week."
Captain Lewis Moody felt England had "done the basics right".
He said: "We looked at our performances (after the Georgia game) and we had a lot to improve on. We did that today, the performance across the board was much better and that's what happens when we keep hold of the ball and do the basics right.
"Keeping them out of our tryline was very pleasing. We had a point to prove to ourselves today and we've gone some way towards doing that."
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