Sunday's 1-0 home defeat by rivals Bristol City left them 14th in the Championship and seven points adrift of the top six after 16 matches.
"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 in a statement on Cardiff's official website.
"It is a shared belief that this is the right time for a new voice as we believe this squad of players is more than capable of getting success," Warnock continued.
"Finally, I would like to thank all you supporters for getting behind me and the team. I hope you have all enjoyed the journey. I am proud to leave the club as a united one and long may you all achieve the success that you deserve. Be lucky."
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!