The phrase "at once left" is correct and usable in written English.
It can be used to indicate that someone left immediately or without delay.
Example: "Upon hearing the news, she at once left the room, unable to contain her emotions."
Alternatives: "immediately departed" or "left right away".
At other times, every feeling hit me at once, leaving me exhausted.
But trying to serve so many "masters" at once leaves little time for themselves and their own careers.
Toni, in whom the wreckage of beauty was still apparent, died almost at once, leaving a young son.
Mr. Bailey works on multiple pieces at once, leaving objects out everywhere around his house and studio until he figures out how he wants to proceed.
Mark comes flying out of one thick-muscled closeout (a wave that breaks all at once, leaving a rider nowhere to go) chattering about needing a longer board.
Schubert worked on too many at once, leaving so many fragments that scholars don't know which piece goes with which.
Analysis suggests that North would have made his contract after a club lead by playing diamonds at once, leaving the trumps alone.
But this decision to abandon their education bill means they've managed two U-turns at once, leaving a trail of chaos behind them.
Different parts of the university were communicating different things, she adds, and all the pitching was taking place "loudly, all at once," leaving some potential students, business partners and research partners confused about the Michigan State brand.
Although the president at the time was George H.W. Bush, one struggles to imagine even our current multitasking (not to say ambidextrous) President signing two different laws at once, leaving to EPA the extraordinary task of deciding which was the real 1990 amendment to the Clean Air Act.
