Warnock played for Chesterfield, Rotherham United, Hartlepool United, Scunthorpe United, Aldershot, Barnsley, York City and Crewe Alexandra, making a total of 326 league appearances in an eleven-year playing career. At Hartlepool he won the club's Player of the Season award in 1972.

After being involved in Sunday League coaching his first full managerial job was with Northern Premier League side Gainsborough Trinity in 1981. Following this he managed Burton Albion and Scarborough. At Scarborough he and Paul Evans, his assistant, won the Football Conference title in 1987, making them the first team to win automatic promotion to the Football League following the abolition of the re-election system.

He had earlier spent time as a coach at Peterborough United, where he met Posh assistant boss Mick Jones, who would become his own long-time assistant. Warnock then became manager of Notts County with Jones as his assistant. Warnock's assistant at Scarborough Paul Evans and ex-Scarborough physio Dave Wilson also joined the backroom staff, and the pair achieved successive promotions to get County into the top flight in 1991 (with Warnock turning down lucrative offers to manage Chelsea and Sunderland during this time), before Warnock left in the 1992–93 after County's relegation had cost them a place in the new Premier League.

In March 1993 he took over as 'consultant' at Torquay United, saving the club from relegation from the Football League. Warnock resumed his partnership with Jones, Evans and Wilson at Huddersfield Town, his appointment coming in July 1993.

Warnock was quick to inject new blood into the Terriers' side, snapping up 'keeper Steve Francis, Darren Bullock, Ronnie Jepson, Tom Cowan and Pat Scully during his first season, all of whom would go on to become mainstays in the 1994/95 promotion season. He also showed faith in Centre of Excellence products such as Chris Billy, Simon Baldry and Andy Booth – a player then struggling to make the breakthrough who would go on to become a club legend in modern times. Despite these acquisitions Town struggled for most of the 1993/94 season, their last at Leeds Road, and Warnock was quick to offload fan favourites Iwan Roberts, Iffy Onoura and Chris Marsden while introducing a more direct style of play. He also didn't enjoy the best of relationships with cult hero Phil Starbuck.

The run to the final of the Autoglass Trophy coincided with an upturn in league form and a mass optimism further bolstered by the move to the new Alfred McAlpine Stadium for the 1994/95 season. Warnock's side won the Yorkshire Electricity Cup in late 1994. Warnock's side were genuine contenders for automatic promotion until falling away in the final few games to finish 5th (the final Play-Off spot that season due to league re-structuring). They triumphed on penalties over 2nd-placed Brentford after two thrilling ties and went on to beat Bristol Rovers at Wembley.

He quit Huddersfield just days after their promotion, but made a swift and surprising return to management at Plymouth Argyle, which had just been relegated to Division Three. Warnock and Jones achieved yet another promotion to get the Pilgrims into Division Two (via the playoffs).

Warnock then left Plymouth, with Jones taking over as Pilgrims boss. Warnock rounded out the 1990s with Oldham Athletic and Bury.

He was appointed as manager of his boyhood club Sheffield United on 2 December 1999. In 2002–03, Warnock led Sheffield United to the semi-finals of the FA Cup and League Cup as well as the First Division play-off final, with the Blades beaten 3–0 by Wolverhampton Wanderers. This was the first time in his management career that he had lost a play-off contest, as he had achieved four promotions via the playoffs in the 1990s.

In 2005 Jones resumed the partnership by taking up the assistant's post at Bramall Lane, and at the end of the 2005–06 season the club were promoted to the Premiership as runners-up in The Championship.