Sorensen joined Saints in 2010 from Newcastle and has already racked up more than 50 appearances for the club.
This season the 32-year-old has featured in all bar two of the Saints' matches in all competitions, scoring his first try for the club in the Heineken Cup clash at Castres.
And the New Zealander is thrilled to have sorted out his future at Franklin's Gardens.
"I'm excited to have signed again," he said.
"Moving to Northampton has been a great move for me, and my family have settled in the area.
"We've been made to feel very welcome by everyone at the club and the Saints supporters. It's a rugby mad area, which is something I really like and reminds me of where I grew up in New Zealand.
"The Saints fans are really knowledgeable about their rugby and give us loads of support home and away.
"It's been exciting to have been a part of the club over the past 18 months, especially playing in a massive match like the Heineken Cup final. It's the type of occasion every player wants to be a part of.
"Hopefully we can get something out of the rest of the season. We're in the LV= Cup semi-finals and are well placed in the Aviva Premiership with plenty to build on. And then over the next couple of years we can take the next step up and I'm looking forward to being part of the Saints squad in the future."
Northampton have also announced that McMillan - who has been on trial at Franklin's Gardens since October - has signed a short-term contract until the end of the season.
McMillan, a former England junior international, makes the switch from Birmingham & Solihull and will cover Brett Sharman, who has been ruled out for several months with a knee injury.
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