Growing up in Tsawwassen, British Columbia, Seabrook played minor hockey with the South Delta Minor Hockey Association. He also played spring hockey with the Pacific Vipers, along with three future Chicago Blackhawks teammates, Colin Fraser, Troy Brouwer and Andrew Ladd. He went on to play major junior in the Western Hockey League (WHL) with the Lethbridge Hurricanes, who drafted by him in the first round of the 2000 WHL Bantam Draft. Seabrook played with future Chicago Blackhawks teammate Kris Versteeg for three seasons in Lethbridge. After a 42-point season in 2002–03 with the Hurricanes, Seabrook was drafted 14th overall by the Chicago Blackhawks in the 2003 NHL Entry Draft.
Seabrook made his professional debut after completing his fourth and final WHL season, being assigned to the Norfolk Admirals, the Blackhawks' American Hockey League (AHL) affiliate, for the final three games of the regular season and six post-season games. Seabrook cracked the Blackhawks' line-up his first year out of junior in 2005–06 and recorded five goals and 32 points as a rookie. In his third NHL season, in 2007–08, Seabrook matched his rookie total with 32 points while tallying a personal-best nine goals. He began the season on the top-defensive pairing with Duncan Keith.
The following season, Seabrook became an integral part of a young and rejuvenated Blackhawks team that made the Stanley Cup playoffs for the first time in seven years the following season in 2009. He scored 11 points in 17 post-season games as the Blackhawks made it to the Western Conference Finals, where they were defeated by the Detroit Red Wings. On March 17, 2010, Seabrook was knocked out by a high hit from James Wisniewski, a former teammate. Seabrook missed the next two games, while Wisniewski was ultimately suspended for eight games. On June 9, 2010, the Blackhawks won the Stanley Cup after defeating the Philadelphia Flyers 4–3 in overtime.
On February 27, 2011, Chicago signed Seabrook to a five-year, $30 million contract extension.
On May 29, 2013, Seabrook scored the Game 7 overtime winner against the Detroit Red Wings in the Western Conference Semi-finals, advancing Chicago to the Western Conference Final. During the Finals, on June 19, Seabrook scored the overtime winner against the Boston Bruins in Game 4 of the series. Seabrook won his second Stanley Cup as the Blackhawks beat the Bruins in six games.
During the first round of the 2014 playoffs, Seabrook delivered a high-hit to David Backes of the St. Louis Blues. Backes was forced to leave the game and Seabrook was assessed a match penalty. The Department of Player Safety reviewed the hit and suspended Seabrook for three games.
On September 17, 2015, the Blackhawks named Seabrook as an alternate captain. On September 26, 2015, Blackhawks extended Seabrook's contract for 8 years.
During his junior career, Seabrook represented Team Canada in two World Junior Championships. In 2004 in Finland, he recorded three points to help Canada to the gold medal game against the United States, where they were defeated 4–3. Returning the following year in 2005 in the United States, he matched his three-point total from the previous tournament and helped defeat Russia 6–1 in the final to capture Canada's first gold medal that began a five-year run.
After his rookie season with the Blackhawks, Seabrook made his senior international debut with Team Canada at the 2006 World Championships in Latvia, where Canada failed to medal.
On December 30, 2009, Seabrook was called and asked to play for Team Canada at the 2010 Winter Olympics in Vancouver, along with Blackhawks teammates Duncan Keith and Jonathan Toews. He was expected to continue his NHL pairing with Keith during the tournament, but he instead became designated as Canada's seventh defenceman. He notched one assist over seven games as Canada won the gold medal over the United States 3–2 in overtime on February 28, 2010.
Seabrook and his wife Dayna have three children: a son Carter Seven (born on August 16, 2013), a daughter named Kenzie Belle (born on June 19, 2015), and another daughter named Dylan Ivy (born on May 19, 2017).
On February 25, 2014, Seabrook, along with Blackhawks teammate Duncan Keith, made a cameo appearance in the episode "Virgin Skin" on the NBC drama Chicago Fire. His younger brother, Keith Seabrook, was drafted by the Washington Capitals and played in the AHL from 2009 to 2012, returning in 2014 after a brief hiatus from the sport.