The Canadian international winger returned from international duty in Morocco to complete his move and will remain in south Wales despite Canda having another friendly to play today in Africa.
Hoilett, 26, has been without a club since his contract ended at QPR in the summer.
Hoilett told Cardiff's official website: "I'm really pleased to be here and I'm ready to go.
"The manager puts belief in his players and this is a great group of lads. Everyone loves playing for the gaffer.
"I'm here to hopefully bring some more creativity and goals. I can't wait to get started."
Warnock added: "Junior did well for me at QPR and by the time I left there he was one of the club's most important players.
"I'm delighted to have him here with us at Cardiff City and I know that he'll respond to the fans and me.
"What's more, he's a player who will help bring the best out of those around him."