The widely-travelled 63-year-old has agreed a contract which commits him to the Elland Road club until the end of next season.
Leeds chairman Ken Bates said: "We believe the appointment is arguably the most important we have made. The objective was to appoint a manager who had for a proven track record of getting teams promoted and in Neil we have a man whose record is second to none."
He added: "We want to be in the Premier League and we will support Neil in the quest to get us there."
Warnock, who will appoint his own backroom staff, told Leeds' official website, www.lufc.co.uk: "I feel I have one big challenge left in me and believe Leeds is a club that should be in the Premier League.
"I want to be the man who is able to deliver this for a set of fans who never cease to amaze me with their numbers and their loyalty. Having met with Ken Bates it was an easy decision to take up the challenge and with his support, we share the same vision of getting Leeds United in the Premier League."
Neil Redfearn remains in charge for Saturday's npower Championship game against Doncaster, and will return to his role with the club's Academy next week.
Ray Fell, chairman of Leeds' supporters club, is pleased with the appointment of a man who has led QPR and Sheffield United into the top flight.
He told Sky Sports News: "He's a man that can motivate us and reopen our season. There's no money available so he may bring a loan player in or two - but he'll give us experience and get the most out of the players on the books, which for the last period hasn't been happening."