Promoted Brest, who have not conceded a goal in their last eight league outings, moved on to 21 points from 11 games after snatching their sixth win of the season courtesy of late goals by Mario Licka and Romain Poyet.
Olympique Lyon, who had a terrible start to the season, jumped from 14th to eighth after youngster Alexandre Lacazette scored to give them a 2-1 home win over Sochaux.
The clash between third-placed Olympique Marseille and second-placed Stade Rennes was postponed because of bad weather.
In Brest, the hosts were made to wait before breaking the deadlock when Czech midfielder Licka headed home from close range in the 79th minute.
Six minutes from time, Poyet curled a fine shot into the top corner to put the result beyond doubt.
Brest, who filed for bankruptcy in 1991, had not featured in the top flight since that time.
They enjoy a surprising six-point lead over Lyon, who revived their campaign against Sochaux despite being without the injured striker Lisandro Lopez.
Brazil winger Michel Bastos's first-half opener was cancelled out by Ideye Brown in the 66th minute.
The 19-year-old Lacazette, who had replaced the injured Bastos shortly before the break and had only played 47 minutes in Ligue 1 in his career, made it two for Lyon after being perfectly set up by Maxime Gonalons.
