Once again defensively brittle, the Dolphins launched a second half comeback to draw within six points of the Rabbitohs, but veteran five-eighth Walker landed his first career 40/20 in his 11th season to change the momentum of the game.
"Cody is a big game player who steps up in the big moments," Rabbitohs captain Cameron Murray said.
"He changed the momentum and it was something we really needed."
When Mitchell went over for his second try after sustained Walker kick pressure, the visitors were home at Suncorp Stadium.
The Rabbitohs suffered a blow on the cusp of halftime when half Ashton Ward left the field with an ankle injury. He was later seen in a moonboot and on crutches, after being replaced by Jayden Sullivan.
Souths' half stocks are depleted with Jonah Glover (broken jaw) and Jamie Humphries (suspended) unavailable.
The Rabbitohs ran in three tries in the final seven minutes of the first half to take control 28-10 at the break, with winger Alex Johnston going in for his 211th career try - just one behind legend Ken Irvine in top spot on the all-time list - after new signing David Fifita performed a twist and spin.
Murray and right winger Campbell Graham were enormous, with both hitting back from injuries that have hampered them.
Having played just one game last year due to an achilles rupture, Murray had his trademark quick play-the-balls going to give his side momentum.
Graham, who played only 11 games in the past two seasons due to a raft of injuries, scored a double in a powerhouse display.Â
Dolphins speed demon Jake Averillo did well to snaffle a Walker pass and race 90m against the run of play, and half Isaiya Katoa set up back-rower Kulikefu Finefeuiaki, but the Rabbitohs dominated the first half.
The Dolphins had a focus on defensive resilience in the pre-season. Its absence on Sunday disappointed coach Kristian Woolf.
"We have got to be better than that," he said.
The decision to play Jye Gray at fullback and Mitchell at left centre bore fruit.
Gray raced 70m before Mitchell carried four Dolphins over the line in a trademark bustle. Gray was also on hand to support hooker Peter Mamouzelos for a try of his own.
"Latrell was happy with that and accepted that and he showed what he can do in the centres," Rabbitohs coach Wayne Bennett said.
"I'd like to think he will play most of the season in the centres."
Rabbitohs prop Keaon Koloamatangi was the best player on the field, running for more than 250m.
South Sydney forward Lachlan Hubner had a mixed bag against his former club, with two of his errors costing tries.
The Dolphins hit back after halftime, with a run and kick by new signing Selwyn Cobbo setting up centre Herbie Farnworth for a try as Hubner bungled the clean up.
Butter fingers by Hubner opened the door for electric Dolphins fullback Hamiso Tabuai-Fidow to streak away.
After Walker's kicking genius, Hubner then crashed over under the posts to take the margin back out to 12, before Mitchell sealed the victory ahead of a late consolation try to the Dolphins' Jamayne Isaako.
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