Reaction was understandably mixed - we've picked out a range of responses to the announcement. Scott McInally wrote: "So final confirmation that we are heading into the past."
Stephen Wray added: "Good luck Neil, now [Celtic], back your man and give him the best chance to succeed at all levels."
Lubomir El Guzman said: "Wasn’t my first choice but it is what it is. Let’s get behind him now."
Another fan backed the appointment: "Come on Neil, prove these doubters wrongs. One Neil Lennon."
Knoxy simply stated: "Thunder = brought back."
One supporter appeared to be against the move, tweeting: "Would anyone like a free season ticket?"
Matt G wrote: "Done now, everyone needs to back him for nine and 10."
Daniel Gallagher wasn't happy: "£9m from Leicester. Three months to prepare and Lennon is the outcome. Can’t say I’m surprised. Cheap appointment and lack of ambition shown once again from the club. Disappointing to say the least."
Michael McCahill had his say: "Don’t think we could have anyone more appropriate to lead us to the “10”. Let’s all get behind our manager - in Lenny we trust."
One Celtic follower wanted to see movement in the transfer window: "Let's get cracking on, signings needed NOW not later."
PaddyBoy backed the decision: "The negativity is shocking. People forgetting this man has a better win percentage then Rodgers... give the man a chance with his own team next season. What kind of so-called fans do we have? You'd swear we appointed Ronny Deila again."
One fan was underwhelmed: "Let the penny pinching commence."
Kyle Smith tweeted the club's official account: "Go home Celtic, you're drunk."
One supporter simply posted an image reading: "Delete that tweet or I will go to police."
Another fan said: "Thanks for ruining my birthday Celtic, appreciate it."
Dr Emmet Brown had clear instructions: "Right, get the tin open and bring in the targets Lenny has set. No nannying about this time. Deliver and give us a good start to the new era. Celtic first, Celtic last and Celtic overall."
Declan Carroll bemoaned the announcement as a "puppet appointment, regardless of what anyone has to say" while another fan replied: "Good joke, now announce Rafa [Benitez]."
Peter Nolan said: "Some of the reaction on here is totally scandalous. Lennon has put up with more than most would to to play for and manage Celtic.If he can get us to 10 there's nobody I'd like to see more doing it. Good luck."
Included among the other replies were Homer Simpson backing into a hedge, several people crying or looking shocked and a gif of Batman saying: "I don't want this."