Match report from Mansfield
Orient's hopes of automatic promotion were brought to an end with a 2-0 defeat at Mansfield, which also saw striker Carl Griffiths dismissed.
Two goals in the last 12 minutes gave Mansfield their first win in seven games and all but ensured their Third Division status next season.
The result also ended Orient's hopes of automatic promotion and the visitors did not help their own cause when leading scorer Griffiths was sent off for an off-the-ball clash with Dave Jervis just before half-time.
Although Mansfield completely dominated the first half, Orient always looked dangerous on the break and Ahmet Brkovic wasted three excellent chances and also found the side-netting after the break.
But teenage substitute Liam Lawrence did the trick when Orient failed to clear Lee Williamson's free-kick and he forced the ball home through a crowded six-yard box.
And, with Orient pushing forward, Mansfield caught them on the break in time added on as Shayne Bradley steered the ball home across the goalkeeper.