Observer editor John Mulholland is leaving the title after ten years in the job to become editor of Guardian US.
A new editor for the title is set to be appointed in early 2018.
Guardian US editor Lee Glendinning is to become executive editor for membership based in London. She previously helped launch Guardian Australia in 2013.
Current executive editor for membership Natalie Henman is to become assistant editor “leading work on new initiatives relating to the mission of the Guardian”.
Viner said: “John Mulholland has forged a powerful identity for The Observer as the paper of ideas and intellectual discourse, at the forefront of the debate over Brexit and with strong opinion writing, coverage of foreign affairs, culture and technology. He will bring similar insights, originality and verve to our Guardian US edition.
“Under Lee Glendinning’s editorship of Guardian US we have published award-winning investigative projects and produced a compelling digital edition during one of the most extraordinary news periods in recent memory, encompassing a Trump election campaign and the first year of his presidency. Having led Guardian teams and got to know Guardian readers across three different continents, Lee is the ideal editor to take Guardian membership to an exciting new phase.
“In just two years Natalie Hanman has taken Guardian membership from a small-scale concept based on events with 12,000 members to a major part of the Guardian’s future with over 800,000 supporters.
“Natalie’s innovative work has helped us to build meaningful relationships with readers and to begin the reinvention of the Guardian’s business model. Her strong understanding of the Guardian’s role in society will be vital in helping to develop the Guardian’s mission in 2018.
“These senior appointments will help to shape the future of Guardian journalism and will be critical to the success of our strategy. I’m delighted that John, Lee and Natalie will be taking on these crucial roles.”
Mulholland said: “Editing The Observer has been a dream job for the last ten years, and it’s of course hard to leave the fiercely talented, loyal and good humoured Observer team, who I will watch with great fondness from afar. I am incredibly excited by the opportunity to lead Guardian US’s distinctive and original reporting.”
The appointments will take effect from Spring 2018.