"I'm happy to be a part of the beginning," Williams said. "I can't really put into words. I'm very blessed."
It has been a banner season for Williams, who spent the previous two years out of the NFL. The former first-round draft pick, by the Detroit Lions in 2005, finished the regular season with a career-best 65 catches for 751 yards and two touchdowns, including the only TD in Sunday night's 16-6 division-clinching win over the Rams.
But Williams proved worthy through his work during the offseason, eventually making Seattle comfortable enough to cut T.J. Houshmandzadeh and make Williams its starting receiver. He quickly became a favorite target of quarterback Matt Hasselbeck, posting three games with 10 or more catches this season.
"I always felt like I needed to be loyal to coach Carroll. He gave me the opportunity," Williams said. "Obviously, there was some excitement about free agency and the possibilities, but I knew I wanted to be here, and if we could get close to where I wanted to be, I was going to make it happen."
Williams said the idea of an extension was first broached around midseason. Williams and his representatives wanted to delay moving forward, but the Seahawks didn't want to risk the receiver becoming a free agent.
Williams signed a one-year deal with a base salary of $545,000 with the Seahawks for the 2010 season.
Financial details of his extension weren't released, but Williams, who will turn 27 on Tuesday, told The Seattle Times they were "enough to put a smile on your face."
"It's a statement that Mike has come back for real, that he is to the point where he has instilled the confidence in us to keep him around for a good while," the coach said. "And he's still just getting started."
The Associated Press contributed to this report.