Ottawa Senators

In the 2008–09 season, Smith signed an entry-level contract with the Ottawa Senators^[3] and attended the team's training camp. He played with the team until September 21, when he was demoted to Ottawa's AHL affiliate, the Binghamton Senators. He was the final cut made by the Senators in the pre-season. He made his NHL debut as an emergency call-up for a November 29, 2008, game against the New York Islanders, registering no points. On January 16, 2010, he scored his first NHL goal, against Carey Price of the Montreal Canadiens. Smith would play in all six of Ottawa's 2010 Stanley Cup playoff games.

On May 20, 2011, Ottawa signed Smith to a two-year, one-way contract.^[4] He spent the 2011–12 season with Ottawa, registering 14 goals and 26 points in 81 games, while also appearing in all seven of Ottawa's 2012 playoff games, registering one assist before as the Senators were eliminated by the New York Rangers in the Eastern Conference Quarterfinal. On 5 September 2012, it was announced Smith signed a four-year contract with the Senators worth an average annual value of $1,887,500.^[5]

Smith would appear in all 48 games of the lockout-shortened 2012–13 season, scoring 4 goals and 11 assists, and would score 1 goal and 1 assist in 10 games during the 2013 playoffs before Ottawa was eliminated by the Pittsburgh Penguins in the Eastern Conference Semifinal. Smith appeared in all 82 games of the 2013–14 season, scoring 13 goals and 9 assists as Ottawa failed to qualify for the 2014 playoffs.

Smith missed most of the 2014–15 season due to dislocating a wrist after a collision with the Boston Bruins' Brad Marchand in December 2014.^[6] He would return to the ice later in the season and was moved across the lines by then head coach Paul MacLean to try to improve Smith's performance, including switching Smith from a winger to a centre.^[7] He would end the season with 2 goals and 1 assist in 37 games. He would also appear in three games during the 2015 playoffs in the Eastern Conference Quarterfinal against the Montreal Canadiens, which the Senators lost in six games.^[8]

Smith's performance drastically improved during the 2015–16 season, where he appeared in 81 games and achieved career highs in goals (25) and points (36). Early in the season, he anticipated having a productive season due to improving his skating during his off time from his wrist injury that occurred in the previous season.^[8] Smith also believed his improved performance was due to playing on the top line with Mark Stone and Jean-Gabriel Pageau and by new head coach Dave Cameron's decision to move Smith from the fourth line centre to the first line left winger.^[9] Smith was also nominated for the 2015–16 Bill Masterton Memorial Trophy for his "perseverance and dedication to hockey".^[10]

Chicago Blackhawks

Having completed his 11th season with the Senators in the 2018–19 season, Smith's tenure with Ottawa ended after he was traded to the Chicago Blackhawks in exchange for Artem Anisimov on July 16, 2019.^[11]

In his first season away from the Senators, Smith began the 2019–20 season on the Blackhawks' fourth-line, making his debut on opening night against the Philadelphia Flyers on October 4, 2019. He was scoreless in his first 10 games with Chicago before collecting two assists against the Vegas Golden Knights on November 13, 2019. Smith made 50 appearances with the Blackhawks, posting 4 goals and 11 points, before suffering a cut from a skate blade to his hand against the Vancouver Canucks on February 12, 2020. It was later announced that Smith would miss the remainder of the regular season, due to a back injury requiring surgery on March 6, 2020.^[12]

Smith announced his retirement from the NHL on September 17, 2021.^[13]