Washington Capitals

Upon concluding the 2013–14 season with the Pats, Stephenson signed a three-year entry-level contract with the Capitals on April 12, 2014. He was immediately assigned to American Hockey League (AHL) affiliate, the Hershey Bears, on an amateur try-out.^[14] He made his AHL debut on April 16, 2014, against the Norfolk Admirals and scored his first career AHL goal on his first shot in the first period.^[15] Following this, he was invited to participate in the Capitals 2014 rookie camp but was returned to the AHL for the 2014–15 season.^[16] During his time at camp, head coach Barry Trotz placed him on a second line alongside Nathan Walker and Kevin Elgestål.^[17] In his first full season in the AHL, Stephenson was used in a checking role where he tallied seven goals and seven assists.^[16]

The following season, Stephenson was again returned to the AHL for the 2015–16 season after attending the Capitals rookie camp.^[16] He earned his first NHL recall on October 15, 2015, after registering one assist.^[18] In his first two games with the Capitals Stephenson averaged nine minutes and 24 seconds of ice time per game and won nine of the 15 faceoffs.^[16] In his second game, Stephenson played on the fourth line alongside Brooks Laich and André Burakovsky.^[19] He played a total of nine games with the Capitals before being returned to the Hershey Bears.^[20] Upon rejoining the Bears, Stephenson continued to produce and concluded the season with a career-high 21 assists and 28 points.^[21]

Stephenson spent the majority of the 2016–17 season with the Hershey Bears, only being recalled for four games at the NHL level. His first recall in January came after he had recorded 23 points in 36 games and was tied for third on the team in assists and fifth in points.^[21] As a result of his limited recalls, Stephenson finished the regular season with a new career-high 38 points through 72 regular-season AHL games.^[22] Throughout the season, he also recorded a six-game point streak between February 3 and February 12, and registered his first three-assist game on November 18. Following his second career-best season, Stephenson signed a two-year, $1.3 million contract to remain with the Capitals organization on June 29, 2017.^[23]

Although Stephenson was originally returned to the Hershey Bears to begin the 2017–18 season, he was recalled on October 24, 2017, after playing six games. Through those six games, Stephenson led the team with five goals and tied for the team lead with six points.^[24] As a result of numerous injuries to the Capitals lineup, Stephenson played on their top line during a 6–2 loss to the Vancouver Canucks on October 26. During the game, he recorded his first career NHL goal against goaltender Anders Nilsson.^[25] As a result of his performance, Stephenson was placed on a line with Lars Eller and Tom Wilson at the end of October which produced five goals and 13 points through six games.^[26] However, his production slowed down as the season continued and he snapped a 28-game goal drought with two goals in a 5–3 win over the Philadelphia Flyers.^[27] Throughout the season, Stephenson became a mainstay in the Capitals' bottom-six forward group and spent time on the penalty kill and in 3-on-3 overtime.^[28] He finished the season playing 87:20 on the penalty kill, the fifth-most amongst Capitals forwards. As the Capitals qualified for the 2018 Stanley Cup playoffs, he was expected to play a major role in their post-season run.^[29]

Stephenson made his NHL playoffs debut during Game 1 against the Columbus Blue Jackets playing on the fourth line but was promoted to the second line mid-series. While playing alongside Nicklas Bäckström, he recorded four points including an assist on Bäckström's overtime winner in Game 5 and a shorthanded goal in Game 6.^[30] Following injuries to the lineup during Round 2, Stephenson was promoted to first-line right wing with Evgeny Kuznetsov and Alexander Ovechkin.^[31] As a member of the top line, Stephenson helped the Capitals qualify for the Eastern Conference final for the first time in 20 years.^[32] The Capitals met the Tampa Bay Lightning in the conference finals, where Stephenson assisted on a goal that helped Washington tie the series. They beat the Lightning in seven games and met the Vegas Golden Knights in the Stanley Cup Finals.^[33] During the playoffs, Stephenson became the first rookie in franchise history to score a playoff short-handed goal, and the first to record two straight multi-point games.^[34] On June 7, 2018, Stephenson and the Capitals defeated the Golden Knights 4–3 to clinch their first Stanley Cup in franchise history. Afterward, Stephenson stated that he would spend his day with the Stanley Cup in Humboldt, Saskatchewan, in honour of the Humboldt Broncos bus crash victims.^[35] On August 24, Stephenson took the Stanley Cup to the highway intersection where the crash happened and met privately with some of the survivors.^[36]

Following their Stanley Cup run, Stephenson registered five goals and six assists during the 64 games 2018–19.^[34] Due to the suspensions of Tom Wilson and Evgeny Kuznetsov, Stephenson earned a spot on the top lines to begin the 2018–19 season.^[37] Throughout the season, head coach Todd Reirden consistently shuffled the fourth line and Stephenson often played alongside new teammates.^[38] Despite a decline in production, he finished the regular season without taking a single penalty. His 64 games without a penalty surpassed the Capitals previous franchise record set by Keith Aucoin in the 2011–12 season.^[39] Stephenson attributed his decline in production to his mental health.^[40] After a disappointing regular season, he competed in six of the Capitals' seven playoff games against the Carolina Hurricanes. He was a healthy scratch for Game 4 before being reinserted into the lineup prior to Game 5.^[41] On his 25th birthday, Stephenson committed his first penalty in over a year by tripping Hurricanes player Saku Mäenalanen in Game 6.^[39] After being eliminated from the playoffs, he signed a one-year, $1.05M contract extension to remain with the Capitals.^[34]

Vegas Golden Knights

On December 2, 2019, having contributed with just four points through 24 games, Stephenson was traded by the Capitals to the Vegas Golden Knights in exchange for a fifth-round pick in the 2021 NHL Entry Draft.^[42] Following the acquisition, the Golden Knights' general manager Kelly McCrimmon admitted that the team had its eyes on Stephenson a while. He made his debut the following day playing alongside Tomáš Nosek and Ryan Reaves, where he scored the opening goal in a 4–3 victory over the New Jersey Devils.^[43] Upon joining the Golden Knights, Stephenson broke out offensively and set new career-highs in goals, assists, and points.^[1]

Following his breakout season, Stephenson signed a four-year, $11 million contract extension with the Golden Knights on October 7, 2020.^[44] He was promoted to the first line with Mark Stone and Max Pacioretty, which outscored opponents 15–5 at five-on-five.^[45] On April 1, 2021, Stephenson was suspended for two games after he elbowed Kings defenseman Tobias Björnfot at 18:07 of the second period.^[46] Following his suspension, he matched his career-high from the 2019–20 season with 26 points in 41 games played.^[47] As the Golden Knights qualified for the 2021 Stanley Cup playoffs, Stephenson missed three games against the Montreal Canadiens with an undisclosed injury.^[48]

Upon completing another breakout season, Stephenson began the 2021–22 season without his usual linemates, who had both been injured early in the season. In spite of this, he led the Golden Knights in scoring with nine points through six games.^[49] Ultimately, Stephenson once again set new career-highs, posting 21 goals and 43 assists for a total of 64 points in 79 games.

After starting the 2022–23 season with 44 points through 51 games, Stephenson was named to his first career NHL All-Star Game, as an injury replacement for Matty Beniers.^[50] Stephenson returned to the Stanley Cup Finals in 2023, now playing for the team he defeated when he was a member of the Capitals in 2018,^[51] and would win his second Stanley Cup with the Golden Knights.^[52] Stephenson became the third player of the expansion era to win a Cup with a team he had previously defeated for it, joining Scott Niedermayer (with New Jersey in 2003 and Anaheim in 2007) and Billy Carroll (with the New York Islanders in 1983 and Edmonton in 1985).^[53]