Willy Sagnol's side took the lead in the 80th minute as Mariano took advantage of poor defending from Ludovic Baal to fire home.
More sub-standard play at the back – this time from the hosts - contributed to Lens' equaliser six minutes later as Cedric Yambere's weak back header allowed Pablo Chavarria in to score.
However, in the third minute of time added on, Maurice-Belay exchanged passes with Isaac Thelin before slotting home, leaving Lens rooted to the foot of the table.
Elsewhere, there was a similarly thrilling finale in Caen's 2-1 win at Nantes.
Trailing with 10 minutes to go after Alejandro Bedoya's opener, Caen levelled through Emiliano Sala's penalty.
Then in the final minute of stoppage time, Thomas Lemar's free-kick from wide on the right crept all the way in to lift the strugglers six points clear of trouble.
