"Brian is a top attack coach and I'm sure he will be a real asset to us," said Johnson.
"The appointment of Brian is great news for the England set up as we look forward to the increased time we have with the players under the new agreement with Premier Rugby Limited.
"His appointment finalises the senior coaching team for next season."
The 41-year-old Australian, who previously worked with Eastern Suburbs, ACT Brumbies, Australia Sevens and Bath Rugby, will join forwards coach John Wells, defence coach Mike Ford, scrummaging coach Graham Rowntree and kicking coach Jon Callard in Johnson's England set up.
"The opportunity to coach at international level comes along rarely in one's career so I'm looking forward to my new position with England, it's an exciting time to be involved," said Smith.
"I would like to thank London Irish for the opportunity the club has given me. I've enjoyed working with the playing and coaching staff and would like to thank all at the club, especially its exceptional supporters, for all the encouragement I've received."