The McGowan government has been accused of undermining St John WA to deflect blame for its own failures across the health system.
Michelle Fyfe will step down as ambulance service CEO from July 12, months before she had been due to finish her contract.
Her departure follows a tumultuous period for St John WA, with a blowout in response times and ramping, criticism from the state government over staffing arrangements and reviews into the deaths of two patients forced to wait for ambulances.
One of the patients waited more than two hours despite her call being categorised as priority one and having a targeted 15-minute response time.
The WA government last month stationed senior health and police officials at St John's headquarters in a bid to improve its performance.
The move came amid growing pressure on the service as more staff were furloughed because of COVID-19 infections, with the health minister criticising St John for not immediately activating a policy allowing close contacts back to work.
The United Workers Union's Fiona Scanlon said it was an opportunity for the service to be put on the right track.
"We are not surprised the CEO has taken the decision to leave a service that has been run into the ground on her watch," she said on Wednesday.
"The CEO has been part of an ambulance service with systemic problems."
Ms Scanlon reiterated calls for the service to be brought back into government hands.
A recent parliamentary inquiry found the government should take over the ambulance service if St John failed to lift its performance in the next five years.
A new contract between the two parties remains under negotiation.
Opposition health spokeswoman Libby Mettam said the government had used St John as a scapegoat after overseeing a surge in ambulance ramping.
"St John have been operating with a hand behind their back when it comes to being able to respond to priority calls across the community," she told reporters.
"They are waiting record hours outside emergency departments because the McGowan government have failed to properly resource (hospitals).
"Given the McGowan government can't run a hospital system, how on earth could they run an ambulance service?".
St John WA chair Shayne Leslie thanked Ms Fyfe for her service during challenging times but said the organisation needed fresh leadership.
Ms Fyfe said she was proud of St John's achievements.
"I have missed many precious moments with my family and I am now looking forward to some time off to spend with my husband, children and grandchildren before embarking on my next challenge," she said in a statement.
Chief operating officer Antony Smithson will act in the role while the board searches for a new chief executive.
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