England international star Phillips has completed his move to City on a six-year deal.
City are paying £42m for the midfielder - rising to £45m with add-ons - but Leeds believe the deal will be worth closer to £50m to them for a player who joined Leeds when he was just 14 years old.
Within seconds of transfer being announced, Phillips quickly issued a message to his former club across his social media platforms, paying particular tribute to the club's fans plus former boss Marcelo Bielsa and current Whites head coach Jesse Marsch.
Phillips also saluted the work of chairman Andrea Radrizzani, chief executive Angus Kinnear and director of football Victor Orta and ended his message with an emotional Whites promise, saying it was "never goodbye" and that "I WILL see you soon."
In a message titled Dear Leeds United, my home and my team, Phillips wrote: "Where do I start?
"First of all, I would like to express what a privilege it has been to play for this amazing club. Since making my debut at 17 it has been a journey any Leeds lad could wish for
"When I was young, I went to see my first ever Leeds game at ER. From that moment I knew that my dream was to put on that white shirt and walk out on that pitch.
"I’d like to thank everyone involved for making Leeds one of the best clubs on earth. Every manager I have played under for showing trust in me and teaching me all I know.
"I’d like to thank Andrea, Victor, and Angus for helping to bring this club back to where it belongs. You guys are a major reason as to why our club is where it is today.
"I cannot mention Leeds without mentioning one special person - Marcelo.
"The best manager I have ever come across. He was a man that gave the club life, he gave everyone belief that we were good enough to return to the Premier League after 16 years.
"Not only did you give the club everything, but you also gave me everything I needed to become the person I am today, on and off the field.
"And now to Jesse, what an unbelievable manager, but what an even better person. You came into this club and gave every player belief in a very difficult season and that’s what kept us up. From the bottom of my heart thank you.
"To all my teammates from my debut to my last game thank you for helping a Leeds lad achieve his dream. I wish everyone all the best for the next season.
"Over the last 4 years me and you boys have shared so many memories, from missing out in the play offs to getting promoted and then surviving relegation.
"You lot are unbelievable and deserve everything.
"Finally to my family, the fans! I hope you guys understand my decision and will accept that I only want to chase my dreams and test myself against and with the best teams and players on the planet. As a young kid I had so many dreams and still do… but playing for you was the one I’ll never forget.
"You are the best fans in the world.
"Good luck to everyone at Leeds United.
"I love you all and will miss you.
"Never goodbye. I WILL see you soon. MOT."