The 29-year-old has been linked with a move away from the club this summer, with Barcelona reportedly interested.
However, the France international put an end to the speculation by signing a new deal until 2017 - something that Mathieu describes as a "very easy decision".
"In my head it was a very easy decision," Mathieu told reporters.
"I wanted to renew with Valencia. We are missing only a few small details, but it is virtually done.
"I want to end my career here and I think that Valencia have understood that. I want to thank the club."
Mathieu joined Valencia from Toulouse in 2009.