The Cherry and Whites put last week’s first loss of the season behind them, despite being 9-6 down at the break.
That scoreline came as a result of a kicking dual between Mercier and Saints’ fly-half Paul Grayson, who hit three first half penalties to Mercier’s two.
The second half however saw Northampton fail to register a single score, Mercier nailing three more penalties to secure the points.
A drop-goal from England scrum-half Andy Gomarsall wrapped up proceedings with three minutes to go, as Nigel Melville’s men proved that they are still the team to beat in England.
GloucesterPens: Marcier 5DG: Gomarsall
Northampton SaintsPens: Grayson 3