Waratahs full back Folau is in his first season in the sport and has only played 12 Super XV games having crossed codes for the second time in December 2012, while Leali'ifano is in line for a long-awaited debut after injury wrecked his chances of featuring last year. No8 Mowen has captained the Brumbies since his move from the Tahs two seasons ago and will now challenge veteran Wyclif Palu for a shot at a starting place against The Lions.
"At the front end of the year I might have suggested it was a step too far for Israel Folau, particularly with his lack of positional experience, but the fact that he's been playing fullback has helped him, because he's been in the perfect place in terms of having to pick up on the dynamics of the game," said Deans.
"(It has also given him) the licence to find ways to inject himself into the game and the fact that he's got that done and the smarts he's shown in getting that done was the evidence we were looking for.
"We know he can play on the end of the chain but, critically, he's got a point of difference capabilities that we believe will be useful to have in the mix."
There are also places for Brumbies wing Joe Tomane - whose only cap came in the defeat to Scotland in the summer - and Waratahs centre Rob Horne but hooker Tatafu Polota Nau and flanker George Smith miss out after picking up serious injuries in the latest round of domestic action.
Rebels utility back James O'Connor is the favourite to wear the No10 shirt after Cooper was overlooked but Berrick Barnes and Leali'ifano are also options at fly-half.
Will Genia is the only scrum-half to be selected but Deans will add at least one more on June 11, with Nic White, Nick Phipps and the returning Luke Burgess all in the running.
Uncapped Brumbies full back Jesse Mogg is a surprise absentee given his fine form this term; inside centre Pat McCabe has not been named as he is currently injured; and Kurtley Beale's off-field troubles mean he is another to miss out as he attempts to resolve his alcohol-related problems.
The squad will assemble in Sydney on June 2, with a further six players set to be added to the group once Super Rugby breaks for the mid-year Test window on June 10.
A final squad of 31 will then be named during the team's second week together on Tuesday, June 11, 11 days before the first Test with the Lions in Brisbane on June 22. Deans will also name his captain on that date, with Reds lock
James Horwill widely expected to be handed that honour despite having missed the entire international season last time out.
"It wasn't an easy group to finalize, and there were a lot of hard phone calls made last night and this morning," added Deans, who will have his favoured players in camp for three full weeks prior to the first meeting with Warren Gatland's tourists.
"Today is not the end of it for players who haven't been named. There are six more places to be filled in the squad while every Super Rugby player in Australia will have the opportunity to play international rugby against the Lions during the tour matches, as will the country representative players. It's an exciting time.
"Time together will be critical. You only have to look back to the experience in 2001, where Australia lost the first Test, to see the consequence of a compromised preparation. We have our own more recent experience of something similar.
"Rod Macqueen (the 2001 Wallabies coach) has since made it clear that despite the build-up they had, which included a warm up game, they were caught short in that opening game.
"We have to make the most of the limited preparation time we have in place in order to avoid a similar result."
Qantas Wallabies squad
Ben Alexander, Adam Ashley-Cooper, Berrick Barnes, Nick Cummins, Dave Dennis, Saia Faingaa, Israel Folau, Will Genia, Liam Gill, Scott Higginbotham, Michael Hooper, Rob Horne, James Horwill, Digby Ioane, Sekope Kepu, Christian Lealiifano, Stephen Moore, Ben Mowen, James O'Connor, Wycliff Palu, Benn Robinson, Rob Simmons, James Slipper, Sitaleki Timani, Joe Tomane