The extension is through 2017, as the Patriots added the five years on to Mayo's existing deal, which was to expire after the 2012 season.
The five new years of the deal, from 2013-2017, have a total value of $48.5 million, according to a league source. Mayo will receive $25 million in bonuses/guarantees as part of the extension.
Mayo has $4.5 million in salary guarantees in 2015, according to the source. He is the first player in the NFL that has guaranteed money in 2015. The deal was negotiated by Impact Sports, which represents Mayo.
Mayo, despite missing two games in October with a sprained knee, ranks third for the Patriots with 68 tackles this season after a year in which he logged a career-best 175 and was voted to his first Pro Bowl. He was the AP's defensive rookie of the year in 2008.
News of the signing comes six days after Mayo pulled in his second interception in as many weeks in a win over the Washington Redskins.
His previous interception, in a win over the Indianapolis Colts, had been the first of his four-year career.
Information from ESPNBoston.com's Mike Reiss was used in this report.