He played in the National Hockey League (NHL) for the Philadelphia Flyers, Carolina Hurricanes, Los Angeles Kings, and Washington Capitals.

In four seasons played with the Flyers, Williams struggled to live up to expectations, as he tried to adapt to the varying systems of three separate coaches – Craig Ramsay (2000), Bill Barber (2000–2002) and Ken Hitchcock (2002–2006).

He broke his left hand in his rookie season (hit by David Tanabe of the Carolina Hurricanes) and had various sprains and strains in his sophomore year.

Williams also suffered a left knee injury on January 18, 2003, when he was hit low by the Tampa Bay Lightning's Brad Lukowich.

[6] Once the season ended, he represented Team Canada at the 2004 IIHF World Championship and signed a one-year contract extension with the Hurricanes.

[10] In Game 3 of the 2006 Eastern Conference Quarterfinals, Williams hit Montreal Canadiens captain Saku Koivu with an errant high stick, severely injuring his left eye.

[11] The injury resulted in Koivu missing the remainder of the series and caused Canadiens fans to boo Williams everytime he touched the puck.

Williams continued his previous season's dominance with nine goals and 21 assists by mid-December before suffering a torn ACL and MCL in his left knee.

[17] On September 17, 2008, Williams was thought to be sidelined for four-to-six months when he tore his right Achilles tendon in an off-ice, pre-season workout.

[20] On March 4, 2009, Williams was traded to the Los Angeles Kings in exchange for Patrick O'Sullivan and the Calgary Flames' second-round draft pick.

During the 2012–13 season, Williams scored 11 goals with 22 assists, as the defending Stanley Cup champion Kings would make another deep playoff run before ultimately falling in five games to the Presidents' Trophy-winning and eventual Stanley Cup champion Chicago Blackhawks in the Western Conference Finals of the 2013 playoffs.

[37] On February 7, 2021, shortly after retiring, Williams was hired as a "special advisor" to Hurricanes general manager Don Waddell.

Williams is the grandnephew of Jerry Toppazzini, a forward who played 12 seasons in the NHL, most notably for the Boston Bruins.

[42] Williams is a dual citizen of Canada and the United States, having acquired American citizenship by naturalization in June 2017.