Maria, who is married to well-known Letterkenny athlete Gary Crossan, was one of four Irish athletes who made the 2:27.00 qualifying time for the marathon.
However only three athletes could qualify.
There was good news for another Letterkenny athlete Catriona Jennings who was chosen to take part in the marathon event.
Brendan Boyce from Milford was also chosen to compete at the Olympic Games in the 50km walk.
Friends of Maria McCambridge say she is devastated by the news.
She will now be allowed time to appeal the decision if she chooses to do so.
“Maria has put so much into this and to reach the time an still not be chosen is heartbreaking,” said the friend.
Selectors had to choose the athletes on a range of criteria including previous performances in the event.