Johnson, 25, played two Super Rugby games for the Queensland Reds before being lured to Glasgow Warriors by Townsend in 2015.
Since taking up the Scotland reins two years ago, Townsend has been impressed by Johnson’s ability to develop his game from raw former Rugby League recruit to an international standard inside centre.
"Sam has got better every season, once he got more used to the demands of professional rugby," Townsend told reporters.
"He hadn’t played that much rugby prior to coming over to Scotland but each time he’s been given an opportunity to play at a higher level he’s done really well.
"I thought he would improve Glasgow. He was the type of player we were looking for, a 12 who can pass and defend well and make good rugby decisions. That is part of the way we want to play rugby with Scotland so he fits in very well with that."
Johnson has been frustrated by injuries this season, but gets a chance to partner Huw Jones in the midfield in the absence of regular number 12 Pete Horne.
"He’s timed his return to playing well and has managed to get some game time before the championship," Townsend said.
Johnson is not the only potential new cap, with hooker Jake Kerr and loose-forward Gary Graham named on the bench.
The inclusion of Graham is notable as the flanker has been part of past England training camps without ever winning a cap.
"Jake has also adapted to our environment really well with us and has earned his place in the squad," Townsend says.
"He’s a hard worker, very solid around the lineout and, given the type of game we could be playing, those factors could be very important.
"Gary has an outstanding work-rate and takes the game to the opposition both in defence and ball carrying."
(Reporting By Nick Said; Editing by Toby Davis)
