Jeremie Aliadiere put the visitors ahead with his seventh goal of the season in the 20th minute. Alain Traore headed home an Enzo Reale cross to make it 2-0 18 minutes later.
Lorient - who had numerous chances in the first half - are on 30 points, five behind leaders Olympique Lyon who visit third-placed Paris St Germain (32 points) later on Sunday (2000 GMT).
St Etienne, whose goal-drought has now lasted for more than 500 minutes, are eighth with 27 points.
Earlier, Henri Saivet rescued a point for 10-man Girondins Bordeaux as he struck home in the 76th minute to earn his side a 1-1 draw at basement side Nancy.
It was the fourth consecutive league draw for Bordeaux, who are seventh on 28 points, and the sixth in a row for Nancy, who wasted a lead in five of those games. Nancy have 11 points. (Reporting by Gregory Blachier; Editing by Mark Meadows)