The 21-year-old joined late on Tuesday night, North End paying a undisclosed fee which is understood to be just north of £1m.
Lilywhites head coach Frankie McAvoy labelled him as 'one of the best young players in Scotland'.
McCann, who agreed the deal from Belfast where he is with the Northern Ireland squad, has signed a four-year contract.
He played against North End for St Johnstone in the pre-season friendly at McDiarmid Park in July.
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Last season, McCann helped the Saints - managed by ex-PNE defender Callum Davidson - win both cup competitions in Scotland.
Born in Edinburgh, he came through the youth system at St Johnstone so it was a big decision for him to leave.
McCann said: "It’s brilliant, it’s been such a long day today. I’m just so buzzing to get it all sorted and get it over the line and I just can't wait to get back and get in with the lads next week.
"St Johnstone are a great club. I’ve been there since I was young so it was difficult leaving because they’ve done so much for me when I’ve been growing up, but an opportunity to move down to a club like this is just too much to pass up and I just can’t wait to get started.
"I try and do every part of the game. I try and put myself around, getting forward, getting back, getting involved.
"I put myself into tackles, pressing high, I also try and chip in with goals at the other end so hopefully I can do that and the fans will take to me which is what I’ll try and do."
McCann made the most number of tackles in the Scottish Premiership last season, with him able to play as a shielding midfielder in front of the back four or further forward in a 'box-to-box' role.
McAvoy said: "It's magnificent, it’s a brilliant signing for Preston North End. He’s one of the best young players in Scotland.
"He’s done remarkably well playing at St Johnstone. He’s a full international with Northern Ireland so we’re absolutely delighted to get the deal done.
"I cannot thank Mr Hemmings, Peter Ridsdale and Craig Hemmings enough for getting the deal done which has been stressful to say the least, but we’ve managed to get it over the line which is brilliant and I’m really looking forward to him coming in and being a part of our squad.
""He's a very dynamic midfield player and I’m sure I’ll add a lot to our squad."
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