The 20-year-old midfielder was on loan at Bundesliga Two outfit SV Hamburg in the first half of this season.
But his opportunities were limited to six league substitute appearances in Germany, where he scored once.
"It is an exciting move and it is good for my development," England Under-21 international Doyle told the Cardiff website.
"I'm looking forward to the challenges ahead. Hopefully we can jump up the table. I've heard a lot about the fans and I'm excited to play in front of them.
"I'm here to learn, help the team, and to add goals and assists too."
The Manchester-born midfielder has also had Champions League experience as a substitute in City's 1-0 win away to Olympiacos, in November 2020.
Doyle was a member of the England Under-17 squad, captained the England Under-20s and made his under-21 debut in September in a 2-0 win over Kosovo.
"He's someone we earmarked and somebody we believe can really help us, so we're really pleased to get it over the line," said Bluebirds boss Steve Morison.
"We've been entrusted by Man City to take one of their players and get him some experience. It works well for both parties. He's an excellent player and one I think the fans will really enjoy watching."
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