The author and journalist said it was the right time to step aside, announcing he will not stand in the lobby group's elections in October and conclude when his term finishes in November.
"I have long stated that I was never going to be the one to lead the movement at the actual referendum," he said in a statement.
"The passing of Queen Elizabeth II confirms that decision, as the whole campaign moves to a new phase.
"A new generation is now stepping up to lead the campaign, many of which were too young to vote when the last referendum was held - some of which were not yet even born."
The Queen's death in September reignited debate over Australia's constitutional independence, but polls have been largely inconclusive.
Federal assistant minister for the republic Matt Thistlethwaite said the federal government's priority remained the referendum on an Indigenous voice to parliament.
"If that's successful, that lays the foundation and is the first stepping stone for the next constitutional amendment, which we hope will be an Australian head of state," Mr Thistlethwaite told Perth's 6PR Radio on Wednesday.
"I'm consulting Australians with their views."
Mr FitzSimons lauded the progress of the republican movement in his seven years as figurehead.
"I am thrilled with what has been achieved by the movement on my watch," he said.
"It is wonderful, right now, to have the republic on the agenda, to have a strong movement with expanding membership and money in the bank – and, most importantly, to have for the first time in history, a 'Minister for the Crown devoted to removing the Crown'."
Former chair Geoff Gallop praised Mr FitzSimons' efforts leading the movement.
"Peter's contribution has been extraordinary, and I am so glad to have drafted him to the cause in 2015," Mr Gallo said in a statement.
"In the words of (Prime Minister) Anthony Albanese at the last Republican dinner in Canberra, while Opposition Leader, 'We all know why we are here. Fitz's leadership'."
36°C