Mayor won the Premiership title with Sale in 2006, scoring a try in the 45-20 grand final victory over Leicester.
"Chris is a talented player who will bring plenty of good attributes to the Saints squad," said Northampton's director of rugby Jim Mallinder.
"He is a strong, elusive centre who reads the game well. Chris also knows what it takes to succeed in the Premiership having won there with Sale. He is a good signing."
Mayor, a former England Under-21 international, has made 17 appearances for the Manchester club this season but he has only started four times in the league.
"This is a great opportunity for me," said Mayor.
"It is a new challenge to take my game forward and develop it. I've worked with Jim Mallinder before and he is an excellent coach.
"Saints are a great club. The players are top class and the stadium and facilities are some of the best around. There is also a fantastic crowd and I'm looking forward to playing for them."
Northampton are unbeaten in National League One this season and are 10 points clear at the top with a game in hand over nearest rivals Exeter.
The Saints signed Ireland international flanker Neil Best last week and they have agreed new contracts with key New Zealand pair Carlos Spencer and Bruce Reihana.