A solid two-way defenseman, Hainsey honed his skills during a two-year stint in the United States National Team Development Program, then spent a year at the University of Massachusetts-Lowell before being drafted in the first round (No. 13) by the Montreal Canadiens in the 2000 NHL Draft.

After spending another season at U-Mass-Lowell, the Bolton, Connecticut, native began his pro career with Quebec of the American Hockey League in 2001. He began the next season with Quebec before making his NHL debut with the Canadiens against the New York Rangers on Oct. 11, 2002.

Hainsey appeared in a total of 31 games for the Canadiens during the 2002-03 and 2003-04 seasons and was claimed off waivers by the Columbus Blue Jackets on Nov. 25, 2005.

In Columbus, Hainsey forged his reputation as a steady defensive force with an offensive touch as he led Blue Jackets defensemen in points in 2006-07 (34) and 2007-08 (32) and ranked second (seven) and first (eight), respectively, on his team in power-play goals.

Hainsey signed a five-year, $22.5 million free agent contract with the Atlanta Thrashers on July 2, 2008, and became one of the NHL's prominent shot blockers, recording team highs of 148 in 2009-10 and 168 in 2010-11.

After 14 NHL seasons, Hainsey made his playoff debut in 2017 and had eight points (two goals, six assists) in 25 games. Following the six-game Final victory against the Nashville Predators, Hainsey was the first player handed the Stanley Cup by captain Sidney Crosby.

As an unrestricted free agent, signed a two-year contract with the Maple Leafs on July 1, 2017.

In two seasons with the Maple Leafs, Hainsey had 46 points (nine goals, 37 assists). He played in his 1,000th NHl game on Nov. 1, 2018, a 2-1 loss to the Dallas Stars. He also had two assists in 14 Stanley Cup Playoff games with the Maple Leafs.

Hainsey signed with the Senators on July 1, 2019. He had 12 points (one goal, 11 assists) in 64 games in his first season with the Senators.