The veteran manager previously said he would not continue beyond the end of his contract, which was due to expire at the end of the season.
However, Warnock is to leave his post with immediate effect with the Bluebirds – relegated from the Premier League last term – languishing 14th in the second tier.
"Today I am leaving my beloved Bluebirds after over three years of which have been some of the best days in my long football career," Warnock said.
"I would like to thank Vincent Tan and Ken Choo for their support; Victoria my secretary who has had to look after me over the years; Kevin Blackwell and Ronnie Jepson, my ever-faithful sidekicks and friends; and all the backroom staff on both the coaching and the medical sides who have worked tirelessly to help us achieve the success we have had.
"A special mention also to all of my players, past and present, who have given me everything. Thank you, you're a great bunch."
Warnock was appointed by Cardiff in October 2016 and led the team in promotion to the Premier League in the 2017-18 campaign.