Chris Ashton crossed twice but it was a man-of-the-match 13-point display from Toby Flood that stole the show for England as they grabbed their first win in Wales since 2003.
But despite watching his Six Nations campaign begin with a bang, Johnson insits England are anything but the finished article just yet.
With the visit of Italy next week now firmly in his mind Johnson said: "I thought the win was a bit ugly but you have got to win those Test matches.
"But there are definitely lots of little things that we can work on. I thought we gave a couple of silly penalties away and with things like that we could be sitting here having lost.
"But we fought to the end and it was great for the guys to get a win.
"I thought Tom Wood had a very very good game but as a team there were lots of things we got wrong at times.
"They (Wales) will say they had chances to win and they did, but it's about finishing it off and we did that. I'll hand out the rollockings on Monday."