Laich started playing minor hockey in his hometown of Wawota, Saskatchewan. During the 1999–2000 season, he was the top scorer and most valuable player for the midget AAA Tisdale Trojans. He was the Runner up for league MVP.
Laich started his junior hockey career with the Moose Jaw Warriors of the Western Hockey League (WHL) during the 2000–01 season. He played in 71 games with the Warriors, scoring nine goals and adding 21 assists. Laich was a dedicated student in Moose Jaw, being awarded the club's Scholastic Player of the Year for the 2000–01 season. After the season, he was drafted by the Ottawa Senators in the sixth round, 193rd overall, of the 2001 NHL Entry Draft. He started the 2001–02 season with the Warriors, but was traded to the Seattle Thunderbirds along with Tomáš Mojžíš for Craig Olynick and Stas Avksentiev. The deal was unpopular with Warriors fans, as Olynick left the club and Avksentiev never reported.
Laich spent the rest of his junior career with the Thunderbirds. He started to develop more of an offensive game, finishing off the 2001–02 season with 58 points in the 47 games he played in Seattle. Laich has said that getting traded to Seattle was a good move for him, with coach Dean Chynoweth providing ample opportunities for him to display his skills. He was named the team's Most Dedicated Player after the 2001–02 season. His offensive game flourished further in his first full season with the Thunderbirds (2002–03), as he finished the season with 41 goals and 53 assists, leading the team. These stats placed him sixth overall in the WHL points race. He was named the WHL's Western Conference MVP. He was also named to the WHL's Western Conference First All-Star Team, and the Canadian Hockey League's Third All-Star Team.
Laich debuted with Team Canada at the 2003 World Junior Championships, which were held in Halifax, Nova Scotia. In a game against Sweden, Laich scored Canada's first goal of the tournament. He scored two goals and added four assists in six games at the tournament, where Canada won a silver medal. According to his father, Harold, his son had always dreamed of playing for Canada. Laich's next experience with Team Canada came at the 2010 World Championships. He scored one goal in seven games as Canada finished seventh. During the tournament in a game against Russia, Laich raised the ire of his Capitals' teammate Semyon Varlamov for slashing and cross-checking Alexander Ovechkin. Varlamov said of Laich's play, "...Brooks Laich was constantly trying to slash and cross-check him (Ovechkin) as painful [sic] as possible. After the game, he certainly wished me luck and victory at the world championship, but the unpleasant impression still remains."
During the 2005–06 season, Laich played in 104 regular season and playoff games between the Capitals and their new AHL affiliate, the Hershey Bears; this tied him for the most games played in the organization with Boyd Gordon. After training camp, he started the season in the AHL, but was optimistic about his chances to play in the NHL, saying, "You've just got to work hard. There's spots up there, guys could go down with an injury, you want to be the first guy called up." He spent most of the season at the NHL level, playing in 73 games and recording 21 points. After the Capitals' season was over, Laich rejoined the Bears for their playoff run and scored eight goals in 21 games while helping the team capture the Calder Cup. Laich played the entire 2006–07 season with the Capitals, scoring eight goals and adding ten assists. After the season, Laich and the Capitals went to salary arbitration, where he was awarded a one-year contract worth $725,000.
The 2008–09 season saw Laich play in all 82 games for the Capitals again, where he set new career highs in goals, assists and points; he scored 23 goals and added 30 assists, breaking the 50-point plateau for the first time in his career. During the Capitals' playoff run, Laich played in 14 games and added seven points. He assisted on the overtime winning goal in Game 6 of the Eastern Conference Semi-Final against the eventual Stanley Cup champions Pittsburgh Penguins.
In 2009, Capitals' Head Coach Bruce Boudreau believed that Laich had not yet reached his full potential, saying, "...I still don't think we've seen the best of Brooks. I think he's going to be a 25-, 30-goal scorer in this league and become a staple for a while." Boudreau also said Laich is considered one of the hardest working players on the Capitals' roster by his coaches and teammates. Part of Laich's inspiration comes from his father, who, during a 34-year tenure as a high school principal, only missed three days of work. Laich plays on the Capitals power play and kills penalties in addition to his ice time at even strength. Laich has been lauded for his leadership and was considered for a more formal role in the team's leadership structure in 2009.
Laich received national media attention when he stopped to help two stranded Capitals fans change their flat tire after a Game 7 loss in the 2010 playoffs. When he came across the woman and her teenaged daughter stranded on the Theodore Roosevelt Bridge, Laich stopped to install their spare and apologized for the club's early loss in the playoffs. For his efforts, Laich was hockey analyst Michael Farber's nominee for the 2010 Sports Illustrated Sportsman of the Year. When asked about the incident, Laich responded, "It's not a big deal. It was just a tire. The lady was stranded on the side of the road, I saw that it was just a lady and her daughter, and I figured my tire expertise outweighed hers."
On June 28, 2011, Laich signed a six-year, $27 million contract extension worth $4.5 million per season.
Prior to the start of the lockout-shortened 2012-2013 playing season, Laich played for the Kloten Flyers of Switzerland's National League. In 35 games for the Flyers, Laich posted 17 goals and a total of 38 points. He also reportedly suffered a groin injury, which limited his play upon his return stateside with the NHL. When play resumed for the 2012–13 season, Laich was limited to nine games.
Except for a minor shoulder injury resulting in him missing 16 games, Laich returned to full health in 2014-15. He finished with seven goals and 20 points in 66 games played.
In the 2015–16 season, Laich's offensive production rapidly declined, while his $4.5 million cap hit stuck out as one of the only adverse aspects of a league-leading Capitals squad. On February 27, 2016, the Capitals placed Laich on waivers for the purpose of sending him to the Hershey Bears of the AHL. The following day, on the eve of the trade deadline, Laich, along with Connor Carrick and a second round selection (Carl Grundström) in the 2016 draft, were traded to the Toronto Maple Leafs, his boyhood team, in exchange for Daniel Winnik and a fifth round pick (Beck Malenstyn) in the 2016 draft. The purpose of the trade was to give the Capitals salary cap relief to use toward younger core player contracts that needed re-signing the following off-season. Capitals GM Brian Maclellan called the trade, when asked, the hardest thing he had done as General Manager of the team. Laich had been the longest tenured active professional Washington-area athlete at the time of the trade, concluding his Service of 12 years with the franchise. In what easily stands as the most emotional trade in Capitals history since Laich was dealt to Washington in a package for Peter Bondra, one Journalist wrote "It's ironic that money is what ultimately forced the Caps to move Laich. This is a guy who loves the game so much, he'd probably play for free."
As a free agent from the Maple Leafs, Laich was unsigned over the summer. He later accepted a try-out contract to attend the Los Angeles Kings' training camp. Despite suffering an early injury, Laich returned to impress in the preseason, however was ultimately released prior to the 2017–18 season. With the Kings suffering from injury through the early stages of the campaign, he returned to the fold in signing a one-year, $650,000 deal on October 19, 2017. On November 25, Laich was placed on waivers by the Kings, and was later assigned to their AHL affiliate, the Ontario Reign. Having recorded 1 assist in 12 games with the Kings and before reporting to the Reign, Laich was placed on unconditional waivers, opting for a mutual termination of his contract on November 27, 2017.