Cameron, 21, who was given his professional debut by Cobblers boss Aidy Boothroyd during his spell in charge of the Sky Blues, arrives at Sixfields as cover for injured captain Kelvin Langmead.
Langmead had been an ever-present in the side until he was forced off with a knee problem against Oxford on Saturday but he is expected to return to action before the end of the campaign.
"I gave Nathan his debut in the Championship," Boothroyd told Northampton's official website. "He was one of the stand-out players of my time at Coventry.
"He is an England Under-19 international and he has all the attributes required to be a top-class defender.
"Sometimes young players lose their way a little and a change of scene can do them the world of good and I have a lot of faith in Nathan's ability.
"For someone of 21 he has a fair bit of experience under his belt. He has had some ups and downs in his career already but I am sure we can help get the best out of him.
"I know all about him. He is powerful, good in the air and quick and I am delighted he is here."
On Langmead's injury, Boothroyd added: "In a way we are relieved because thankfully the injury is not as bad as we feared it could be.
"The surgeons say Kelvin will be back before the end of the season and knowing Kelvin he will be back even quicker than they expect. Kelvin has played a huge part in turning this club around and he will continue to do so as we carry on taking this club forward."