The 24-year-old has agreed a four-year contract running until summer 2030, becoming Brian Barry-Murphy’s second summer signing.
Moylan enjoyed an outstanding campaign with Lincoln last season, scoring 11 goals and providing seven assists as the Imps secured automatic promotion to the Championship alongside Cardiff.
His performances earned him a place in the League One Team of the Season and a Player of the Season nomination.
“I’m absolutely delighted to be here. It’s such a massive football club – in Ireland growing up, you watch Cardiff City as a Premier League club, so I’m absolutely over the moon to be here,” said Moylan.
“There were a lot of factors. It’s such a big Club, such a nice city, and Brian (Barry-Murphy) has such a good reputation. The way he plays football makes me think I’ll really enjoy it. I’ve watched the team a lot over the last year, including last weekend's game against Roma, and the football speaks for itself.
“Last year I played a lot in the ten and off the left for Lincoln - I naturally drift to that side. I think the shape here will really suit me, because it’s a footballing team who like to be versatile. When you’re playing with such a good team, surrounded by players who are on the same wavelength as you, it’s going to be really enjoyable.”