The position was originally offered to Mike Ruddock, but the Wales head coach has decided he needs to be with the under-21 side at their World Cup in Argentina.
Johnson, an Australian, was recruited by former Wales coach Steve Hansen and played an integral role in the development of this year's Grand Slam-winning side.
Lions head coach Sir Clive Woodward said: "It's great news that Scott Johnson is going to be in New Zealand with a watching brief for the WRU.
"I saw him close up when I was with Wales during the Six Nations.
"He is a terrific coach and he is also someone who understands the game in the southern hemisphere."
Johnson is looking forward to sharing opinions with Woodward's coaching staff that might help Wales in the defence of their RBS Six Nations crown.
"It will be great to witness 'our boys' working within a different environment other than our own," said Johnson.
"To see them grow and develop under a different set of people will be of great benefit to myself and the Welsh players and management.
"The tour is the culmination of all that is good about rugby in the home nations and being able to share opinions and points of view with people from different approaches and backgrounds can only be of huge benefit, not only to myself but ultimately it will also help the development of Welsh rugby.
"If there's anything I can give to the tour management team by way of knowledge of southern hemisphere rugby and of the New Zealand coaches, I will, of course be forthcoming."