Diminutive midfielder Valbuena struck from inside the box after six minutes in a stuttering performance by the Provence side, who have 71 points with four games left.
Second-placed AJ Auxerre, on 66 points, won 3-0 at Toulouse earlier on Sunday thanks to two goals from Poland striker Ireneusz Jelen.
Marseille and Auxerre meet at Auxerre's Abbe Deschamps stadium on Friday.
"That is why we had to win tonight," Marseille coach Didier Deschamps, the last OM captain to claim the league title, in 1992, told reporters.
"We could have killed it in the first half. We suffered after the break, we had some problems keeping the ball. There were some hot moments but we won, that's what matters."
The St Etienne keeper was helpless at the half hour with another Niang header but the Senegal striker's attempt struck the post.
St Etienne's attacks were easily thwarted by the impressive Marseille centre-back Stephane M'Bia but Marseille's game deteriorated after the interval as Les Verts stepped up a gear.
A few minutes later, a Yohan Benalouane attempt hit Steve Mandanda's post as Marseille eventually held on.
Auxerre's victory was far more convincing.
"We had five very tough minutes, when we could not get out of our half of the field and barely managed three passes in a row," Auxerre captain Benoit Pedretti told French TV Channel Foot+.
"Then, we waited for counter-attack opportunities and set-pieces. We made the most of our chances."
Jelen put the result beyond doubt with 11 minutes remaining, latching on to Valter Birsa's cross from the left.
