The 67-year-old has been the Millers' first choice to replace the sacked Neil Redfearn and his only brief is to deliver Sky Bet Championship safety.
Warnock's appointment came after brief discussions on Thursday night hours after chairman Tony Stewart had revealed there was a three-man shortlist.
Former Sheffield Wednesday manager Stuart Gray and former Blackburn boss Gary Bowyer were the other two man in the running but Warnock was a clear choice.
It was thought that the decision to sack Redfearn was a knee-jerk reaction to Saturday's damaging defeat to Bolton, but it has since emerged that Warnock's representative Mickey Walker was sat next to Stewart at the Macron Stadium.
Warnock, who has vast experience of managing in the Championship, is expected to bring trusted assistant Kevin Blackwell into the club.
He will be unveiled to the media on Friday afternoon and although Nicky Eaden will continue to prepare the team for Saturday's clash with Birmingham it is expected that Warnock will take his place in the dugout.
BREAKING: Neil Warnock has agreed a deal until the end of the season. #rufc pic.twitter.com/mGphHxqbLb— Rotherham United (@OfficialRUFC) February 11, 2016
He faces arguably one of the toughest tasks in his career to keep the Millers in the division and faces a baptism of fire over the next six weeks as the game with Birmingham is followed by clashes against other promotion candidates in Burnley, Sheffield Wednesday, Middlesbrough, Derby and Ipswich in the next eight games.
A brief statement on the Millers' website said: "Rotherham
United are delighted to confirm Neil Warnock as the club's new first team
manager.
"Warnock has agreed a
deal until the end of the season."
