Lee Glendinning (pictured) has been appointed as editor of Guardian US.
She replaces newly-appointed Guardian News and Media editor-in-chief Katharine Viner.
Glendinning has been deputy editor of Guardian US since late last year.
She was also involved, alongside Viner, in the 2013 launch of Guardian Australia.
She began her journalism career at the Sydney Morning Herald before she joining The Guardian in 2006 as a reporter.
The Guardian said that further senior editorial appointments will be made in the coming weeks. It said in a statement that Observer editor John Mulholland has also been made assistant editor of The Guardian with a brief to develop "live journalism".
Last week, as Alan Rusbridger left the title after 20 years as editor to be replaced by Viner, website editor and former Guardian US editor Janine Gibson also departed.
On the latest appointment, Viner said: “Lee was an integral part of the success of Guardian Australia from the beginning, and has had a similarly dramatic impact on Guardian US, bringing excellent news judgment, digital flair and personal tenacity as well as strong management skills.
"Having worked very closely with Lee for the last two years, I know that she will make Guardian US an even bigger success."
Glendinning said: “It is an incredible privilege to lead Guardian US as we continue to cover the stories that matter to our American audience and to deepen our engagement with readers around the world.
"I’m honoured to follow Kath in this role and to work with such an inspiring, dedicated and talented team of journalists. I really feel there is no limit to what the team at Guardian US can achieve.”
According to The Guardian, its US website has "almost 20 million unique users, over a third of The Guardian's total global readership".