Mills had four years left on his £40,000-a-week deal at Leeds, who will continue to pay him £10,000 a week, while City will fork out the other £30,000 - so Mills is not taking a pay cut at all, as reported by some sources.
Kevin Keegan admitted: "The Leeds end held things up for a day but he is ready to train. He is a very fit lad who is ready to go."
But, speaking to the Evening News, he added: "To get a player of Danny Mills' quality on what is basically a free transfer is the sort of thing that doesn't happen very often."
City have been able to finance the move because, like Leeds, they have raised money through securitisation, borrowing £30m against future ticket sales. And it's this, coupled with a long-term commitment of more than £7.5m, that puts the club's financial security further in doubt.
Meanwhile, the tough-tackling defender told BBC GMR: "I'll come here and play in my usual way.
"I'll give it everything, even though I know I sometimes overstep the mark and get into a bit of trouble.
"That is me. I am not going to change and hopefully the fans will appreciate that."