The midfielder has been training with the Cobblers since late 2009 and was offered a deal upon becoming 18.
Built featured for New Zealand at the 2009 under-17 World Cup, scoring in his side's opening game.
"Michael is a talent, he plays wide on the right-hand side of midfield and he is one we are excited about," said Northampton boss Ian Sampson.
"He has been with us for more than a year and now we have received his international clearance we are able to sign him on a professional contract," continued Sampson.
"He is a good player. His family have made a sacrifice to allow Michael to pursue a career in the UK and we think he has a good chance of having a decent career."
Source: BBC Sport
Source: BBC Sport