The Ballymena & Antrim athlete clocked 56.06 - 0.14 seconds inside the Rio mark - at a meeting in the Belgian city of Heusden on Saturday night.
However, the IAAF's deadline for entries was last Monday so McMahon, 24, appears likely to miss out.
McMahon's time cut 0.36 off her previous personal best set last month.
The Northern Irishwoman finished second at the meeting behind Australia's Lauren Wells who won in 55.69.
Christine McMahon is hoping to earn a late call-up to Ireland's Olympic athletics team after running the 400m hurdles qualifying standard.