Kühnhackl played youth hockey for the Landshut Cannibals program for three seasons beginning in 2007 alongside Tobias Rieder. He made his professional debut during the 2008–09 season, playing 42 games with Landshut's professional team in the 2nd Bundesliga, the second-highest level in Germany. He debuted in the Deutsche Eishockey Liga during the 2009–10 season, playing four games with the Augsburger Panther. Kühnhackl was selected 24th overall in the 2009 Canadian Hockey League Import Draft by the Ontario Hockey League 's Windsor Spitfires. He intended to join the team for the 2009–10 season, but a series of injuries kept him in Germany. In May 2010, he signed a contract with the Spitfires to join the team for the 2010–11 season. Ranked as the eighth-best European skater of the 2010 draft class by the NHL Central Scouting Bureau, Kühnhackl was selected 110th overall by the Pittsburgh Penguins in the 2010 NHL Entry Draft. Kühnhackl was happy to be drafted by Pittsburgh, as he listed the Penguins as his favorite team. On 22 March 2011, he signed a three-year, entry-level contract with the Penguins worth $1.83 million. Kühnhackl finished the 2010–11 season with 39 goals and 29 assists for 68 points in 63 games with Windsor. He played four games with the Spitfires to start the 2011–12 season, recording one goal and three assists, before he was traded on 2 November 2011 to the Niagara IceDogs in exchange for Phillip Di Giuseppe, Jaroslav Pavelka, Niagara's second-round picks in 2012, 2014 and 2015 and Niagara's first-round pick in the 2013 CHL Import Draft. On 4 November 2011, Kühnhackl delivered an elbow to the head of Kitchener Rangers defenceman Ryan Murphy and was assessed a five-minute major for charging and game misconduct. In a website video, the OHL noted that Kühnhackl made contact with the opponent's head and hit a vulnerable, unsuspecting player. The OHL also considered the speed and distance travelled when making its decision and noted that Murphy was injured on the play. On 8 November 2011, OHL president David Branch announced that Kühnhackl would be suspended for 20 games as a result of the hit. Kühnhackl made his professional debut in the 2012–13 season with the Pittsburgh Penguins' American Hockey League affiliate, the Wilkes-Barre/Scranton Penguins, but was sidelined by injury after only 11 games. On 2 December 2012, Kühnhackl dislocated his shoulder, requiring season ending surgery. On 20 February 2016, Kühnhackl scored his first NHL goal, a shorthanded breakaway goal during a game against the Tampa Bay Lightning. On 14 March, after recording 6 points in his first 28 games, including a two-assist game against the New York Rangersthe day before, Kühnhackl signed a two-year contract extension with Pittsburgh, along with fellow breakout players Scott Wilson and Bryan Rust. Kühnhackl had an impressive run with the Penguins in the 2016 Stanley Cup playoffs, defeating the New York Rangers, Washington Capitals, Tampa Bay Lightning and San Jose Sharks to clinch the Stanley Cup. He became the third German-born NHL player to win the Cup. In 2017, after the Penguins repeated as Stanley Cup champions, Kühnhackl became the first German-born NHL player to win the Stanley Cup twice with the same team. After the conclusion of the 2017–18 season, his sixth season within the Penguins' organization, Kühnhackl was unable to agree to terms on a new contract as a restricted free agent. On 25 June 2018, he was not tendered a qualifying offer and was released to explore the free agent market. On 2 July, as a free agent, he signed a one-year contract with the New York Islanders.
Kühnhackl was born and raised in Landshut, Bavaria, and first began to play hockey at age two. His father, Erich, played in the top German leagues from 1968 until 1989 and appeared in five Winter Olympics for West Germany. He was named Germany's ice hockey player of the 20th century in 2000. Kühnhackl has been a lifelong fan of the Pittsburgh Penguins, the team that drafted him. He got his start in hockey at age five, following in his family's footsteps—his brother, sister and dad all played hockey in Germany.
