After two seasons of junior with the Lincoln Stars of the United States Hockey League, Backes was selected 62nd overall by the Blues in the 2003 NHL Entry Draft. Following his draft, he joined the Minnesota State Mavericks men's ice hockey team of the Western Collegiate Hockey Association, spending three seasons in the college hockey ranks. Forgoing his senior year with the Mavericks, he turned professional with the Blues, joining their American Hockey League (AHL) affiliate, the Peoria Rivermen. Midway through the 2006–07 season, Backes was called up to the NHL and secured a roster spot with the Blues. Following his fifth season with St. Louis, he was chosen as team captain.
Internationally, Backes represents the United States. He is a two-time Olympian, winning silver at the 2010 Games in Vancouver, and has played in three IIHF World Championships.
Backes played three seasons of prep hockey with Spring Lake Park High School in Spring Lake Park, Minnesota, graduating in 2002. As a senior, he was named All-Conference, All-Metro, All-State and was a finalist for the 2002 Minnesota Mr. Hockey award. He was joined on the Star Tribune's All-Metro First Team with future college teammate, Travis Morin of the Dallas Stars. His high school team included future Minnesota Wild forward, Jarod Palmer. His number 5 has been retired by the school and hangs at Fogerty Arena in Blaine, Minnesota.
After spending parts of two years in junior hockey with the Lincoln Stars of the USHL, Backes was selected in the second round, 62nd overall, by the St. Louis Blues in the 2003 NHL Entry Draft. Prior to joining the Blues, he played three seasons of college hockey at Minnesota State University, Mankato, where he was named to the 2005–06 Men’s RBK Division I West All-America Second Team and the 2005–06 ESPN The Magazine Academic All-America First Team. Immediately after his third and final college season, he was signed by the Blues and assigned to the Peoria Rivermen of the AHL for the final 12 games of the 2005–06 season.
Following his first NHL training camp in September 2006, Backes was reassigned to the Rivermen, remaining in the AHL for the start of the 2006–07 season. Several months into the campaign, he was called up by the Blues and played his first NHL game against the Pittsburgh Penguins on December 19, 2006. Forty-four seconds into the game, Backes registered his first NHL point, a second-assist on a Doug Weight goal, giving the Blues a 1–0 lead. St. Louis went on to win the game 4–1, while Backes recorded 10 minutes of ice time. The following game, two days later, Backes scored his first NHL goal 10 minutes and 47 seconds into the first period versus the Los Angeles Kings. The goal came on a backhand shot past Kings goaltender Dan Cloutier, helping the Blues to a 5–2 win. A month into his initial stint with the Blues, he was reassigned to the Rivermen on January 21, 2007, for three days, after which he was recalled. Spending the remainder of the season with St. Louis, he completed his rookie season with 23 points (10 goals and 13 assists) in 49 games, while also recording 13 points (10 goals and three assists) in 31 games with Peoria. Among NHL rookies who had played at least half the season, Backes ranked 13th in points-per-game at .47.
In 2007–08, he completed his first full season in the NHL, improving to 31 points (13 goals and 18 assists). Becoming a restricted free agent in the off-season, Backes signed a three-year, US$7.5 million offer sheet with the Vancouver Canucks on July 1, 2008. The Blues quickly matched the offer, keeping Backes in St. Louis through the 2010–11 season. With a new contract, Backes tallied 54 points, including a career-high 31 goals, in 2008–09, finishing second behind former Bruins winger Brad Boyes among Blues scorers. He added 165 penalty minutes while playing in all 82 games for the first time in his career. During the season, he scored a career-high four goals on April 2, 2009, in a 5–4 win over the Detroit Red Wings. Backes' efforts helped the Blues reach the playoffs for the first time since 2004. Matching up against the Vancouver Canucks in the first round, the Blues were swept in four games. Backes recorded a goal and two assists in that span.
In 2009–10, Backes' offensive production decreased to 17 goals and 31 assists for 48 points in 79 games. Following the retirement of Keith Tkachuk in the off-season, Backes took on the role of the alternate captain for 2010–11, the last season of his contract. A month into the campaign, on November 12, 2010, Backes signed a five-year, $22.5 million contract extension, keeping him with St. Louis through the 2015–16 season. Leading the Blues in scoring midway through the season, he was named to his first NHL All-Star Game in 2011. As the NHL adopted a draft to determine the teams for the first year, Backes was selected by Team Staal 33rd overall among 36 players. He recorded three assists in a losing effort, as Team Lidstrom won 11–10. Backes completed the season matching his personal best total of 31 goals, while adding 31 assists for a career-high and team-leading 62 points over 82 games. With a +32 rating, Backes came within one point of leading the league in plus-minus, behind Boston Bruins defenseman Zdeno Chára.
Late in the 2010–11 season, Blues captain Eric Brewer had been traded away to the Tampa Bay Lightning, leaving the leadership position vacant for the remainder of the campaign. During the off-season, on September 9, 2011, Backes was chosen to succeed Brewer, becoming the 20th captain in team history. He scored 24 goals to go with 30 assists in 82 games played during the 2011–12 NHL season.
After ten seasons with the Blues organization and leading the club as captain for the last five seasons, Backes left as a free agent after the 2015–16 season. Backes agreed to a five-year, $30 million deal with the Boston Bruins on July 1, 2016.
On November 22, 2016, the Bruins as a team reached the milestone of 20,000 goals all-time in NHL play, dating back to their 1924–25 debut season when left winger Smokey Harris scored the Bruins' first-ever goal on December 1, 1924, Backes' power play goal for the Bruins got them to the 20,000th-team goal total plateau in a 4–2 home rink loss to his previous team, the St. Louis Blues. The 20,000th goal plateau has only been attained by one other NHL team—the Bruins' top rival—the Montreal Canadiens. He is an alternate along with Patrice Bergeron for home games at the TD Garden
Following his rookie NHL season, Backes was named to the United States' national team for the 2007 IIHF World Championship. He played in seven games, contributing a goal, two assists and six penalty minutes, as the U.S. finished in fifth place. He again suited up for Team USA at the 2008 IIHF World Championship, contributing an assist and 35 penalty minutes over six games as the U.S. finished sixth. Competing for a third consecutive year at the World Championships in 2009, Backes registering a goal and four assists in nine games. The U.S. placed fourth, losing to Sweden 4–2 in the bronze medal game.
On January 1, 2010, Backes was named to the United States' roster for the 2010 Winter Olympics in Vancouver. He recorded a goal and two assists over six games, helping the United States to the gold medal game. They were defeated 3–2 by Canada in overtime, earning silver.
On January 1, 2014, Backes was named to the United States's roster for the 2014 Winter Olympics along with St. Louis teammates Kevin Shattenkirk and T. J. Oshie. Although the U.S. team did not win a medal, the forward played in six games, scored three goals, had one assist, and four points — while accumulating six penalty minutes — in Sochi.
Backes got his private pilot license in 2012 and has flown many planes since. He graduated with an applied organizational studies degree with a nonprofit leadership emphasis from Minnesota State University, Mankato in 2014, and was formerly studying to become an electrical engineer.
Backes is a passionate advocate for animal welfare, and has had pets since he was a child and when he was struggling to get a break in the NHL. Together with his wife, Kelly, he is highly active in animal rescue efforts and pet adoption drives. In November 2013, they founded Athletes for Animals, "a united team of professional athletes with a shared passion for rescuing and protecting the welfare of homeless pets nationwide."
On April 4, 2017 six cattle that escaped slaughter in north St. Louis and were put under the care of The Gentle Barn, an animal sanctuary with facilities in California and Tennessee with David providing the funding through GoFundMe for their freedom.