King leaves Haymarket on Wednesday (14 Nov) to become the communications director at The O2, and will be starting in his new role on Monday 3 December.
He took over as editor of Event in 2008 after helping to launch RSVP in 2003.
In 2010, he was appointed editor of Media Week, running both Event and Media Week simultaneously across their print, digital and live event portfolios.
King said: "It has been a fantastic journey at Haymarket over the last nine years, particularly with Event. I have met some amazing people from the industry and I believe Event has played a major part in championing the sector to the wider business community, and making heroes out of all of those that work within it.
"The event industry will always have a special place in my heart, and although it is sad to be leaving I am very much looking forward to the challenges that await me in my new role. I have taken the brand as far as I can, so it is the right time to move on and I believe I can look back with pride on what we have achieved."
"I leave behind a great team - Chantelle, Rachel and Louise - who will continue to evolve the brand and ensure it remains the industry's number one title.
"Finally I would like to say thank you to everyone that has supported me, the brand and the team over the last nine years."
The last print publication under King's editorship will be the November/December issue, which is available from Friday 23 November and can be .