Talks were concluded with the Yorkshireman - who parted company with the Millers in the summer after a memorable short-term stint - on Tuesday night and he was sworn in as Bluebirds boss earlier this morning.
Warnock, who has achieved seven promotions in his 35-year managerial career which has also taken in spells at the likes of QPR, Crystal Palace, Notts County, Plymouth Argyle, Bury and Scarborough, takes over from Paul Trollope, who was sacked by owner Vincent Tan on Tuesday morning after just five months in the job.
The 67-year-old will be joined by his backroom staff of Kevin Blackwell and Ronnie Jepson at the Cardiff City Stadium.
His brief is likely to be similar to the one which he faced at Rotherham when he arrived on a short-term deal in February - ahead of orchestrating a Great Escape as the club staved off relegation.
Since May, the Sheffielder has been linked with a host of jobs including vacancies at Blackburn Rovers, Derby and most recently Aston Villa.
Cardiff sit second bottom in the Championship with eight points, having won just two of their opening 11 league games.
The Welsh outfit will represent the 13th professional club who Warnock has managed.
Warnock’s first match in charge will be a televised home game with Cardiff’s neighbours Bristol City a week on Friday with the Bluebirds then hosting Sheffield Wednesday on Wednesday, October 19.
Warnock will lock horns with his former club Huddersfield in the Principality on November 19.