Johnson has been included in the 23-man matchday squad for the title showdown with Warren Gatland's champions and will feature among the replacements when the team is announced on Friday morning.
A back row vacancy was created when number eight Billy Vunipola was ruled out of the remainder of the Six Nations with an ankle injury sustained in the 13-10 victory over Ireland.
Ben Morgan will take Vunipola's place in the starting XV with Johnson, who has five caps, supplying cover from the bench.
"Tom Johnson has trained with us throughout the Six Nations and we are looking forward to his contribution," England head coach Stuart Lancaster said.
"The performance against Ireland has given us a lot of confidence going into this game against a high quality Welsh team.
"We feel it's right to give the bulk of that 23 another opportunity."
Manu Tuilagi will continue to train with England as his club Leicester are not involved in LV= Cup action this weekend.
Tuilagi only made his comeback from a torn pectoral that has kept him sidelined for six months when the Tigers thrashed Newcastle at Kingston Park on Sunday and has not been considered for selection.
Wasps prop Matt Mullan has also been retained with the squad.
"With Leicester and Wasps not involved in the LV= Cup semi-finals, we are able to keep Matt as cover in the front row and also to give Manu more time to get up to speed with the team," Lancaster said.
England matchday 23 to face Wales at Twickenham on Sunday:
Forwards (13): D Attwood (Bath Rugby), D Hartley (Northampton Saints), T Johnson (Exeter Chiefs), J Launchbury (London Wasps), C Lawes (Northampton Saints), J Marler (Harlequins), B Morgan (Gloucester Rugby), C Robshaw (Harlequins), H Thomas (Sale Sharks), M Vunipola (Saracens), D Wilson (Bath Rugby), T Wood (Northampton Saints), T Youngs (Leicester Tigers).
Backs (10): M Brown (Harlequins), L Burrell (Northampton Saints), D Care (Harlequins), L Dickson (Northampton Saints), O Farrell (Saracens), G Ford (Bath Rugby), A Goode (Saracens), J May (Gloucester Rugby), J Nowell (Exeter Chiefs), B Twelvetrees (Gloucester Rugby).
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- Billy Vunipola
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