Johnson will play his first game of the season on Sunday, just days after being named as one of three uncapped players in Gregor Townsend’s 40-man Scotland squad for the Autumn Test series.
The centre picked up an injury in the pre-season match with Northampton Saints but has recovered enough to be named in Dave Rennie’s starting line-up.
The Glasgow head coach was full of praise for the Australian-born midfielder last year and said ahead of Sunday’s match: “Sam was excellent for us last year and has worked hard to recover from an injury he picked up in pre-season.
“He’s got a really good skillset and communicates well and he’s excited to get back out there.
Johnson is joined in midfield by fellow Scotland squad member Huw Jones, with DTH van der Merwe and Lee Jones starting on the wings. Seymour slots in at full-back on his return to the starting fifteen.
Rennie has stuck with the same forward pack that started against Saracens while Scott Cummings is named among the replacements to cover the second-row.
The Glasgow head coach is looking for a similar performance from his charges in front of a likely sell-out crowd at Cardiff Arms Park.
He added: “We were happy with our physicality and the effort we saw against Saracens and we’ll need to be just as aggressive against Cardiff who picked up a good win in France in Round 1.”
Alex Dunbar, Zander Fagerson, Siua Halanukanuka, Stuart Hogg, Kiran McDonald and Tim Swinson are all out through injury.
John Mulvihill has made four changes to Blues, as Nick Williams replaces the injured Josh Navidi at Number 8, George Earle returns to the second-row, Rey Lee-Lo links up once more with Willis Halahalo in the midfield and Aled Summerhill is promoted from the bench in place of the injured Owen Lane.
Glasgow Warriors team to play Cardiff Blues in the Heineken Champions Cup at Cardiff Arms Park, Sunday 21 October, kick off 3.15pm - live on BT Sport 3.
Starting XV: Tommy Seymour, Lee Jones, Huw Jones, Sam Johnson, DTH van der Merwe, Adam Hastings, Ali Price, Oli Kebble, Fraser Brown, D’Arcy Rae, Rob Harley, Jonny Gray, Ryan Wilson, Callum Gibbins, Matt Fagerson.
Replacements: George Turner, Alex Allan, Petrus du Plessis, Scott Cummings, Chris Fusaro, George Horne, Pete Horne, Niko Matawalu.
Cardiff: Gareth Anscombe, Aled Summerhill, Rey Lee-Lo, Willis Halaholo, Jason Harries, Jarrod Evans, Tomos Williams, Rhys Gill, Kristian Dacey, Dillon Lewis, George Earle, Josh Turnbull, Olly Robinson, Ellis Jenkins, Nick Williams.
Replacements: Ethan Lewis, Brad Thyer, Scott Andrews, Rory Thornton, Samu Mamoa, Lloyd Williams, Garyn Smith, Matthew Morgan.