In 2010, Clifford was among those invited to the Canada men's junior team training camp preparing for the 2010 championship. However, Clifford earned a roster spot with the Kings for the 2010–11 season. On December 9, he scored his first career goal in a 2-1 win over the Calgary Flames. His continued spot on the Kings prevented him from joining Team Canada for the 2010 championships. He primarily played on the fourth line role that could occasionally fight fight. In his second season, the Kings won the 2012 Stanley Cup, defeating the New Jersey Devils in six games. Clifford only appeared in three postseason games.
Clifford joined the Ontario Reign of the ECHL during the 2012–13 NHL lockout, appearing in nine games. He returned to the Kings after the lockout, scoring 7 goals and 14 points in the abbreviated season and earned a new two-year contract with the Kings at the end of the season. The Kings once again won the Stanley Cup in 2014, defeating the New York Rangers in five games. Clifford (alongside Tyler Toffoli) assisted on the championship-winning goal scored by Alec Martinez in double-overtime. He appeared in 24 postseason games, scoring one goal and seven points. During the 2014–15 season he signed a five-year extension with the Kings.
During the 2018–19 season, while in his ninth season with the Kings, Clifford appeared in 72 games, producing a career high 21 points. His tenure with the team ended on February 5, 2020, as Clifford and Jack Campbell were traded to the Toronto Maple Leafs in exchange for Trevor Moore and third-round picks in 2020 and 2021.
During 7 of his 10 seasons with the Kings, he led the team in penalty minutes, with his rookie season of 141 being his highest total.
| Year | Team | Event | Result | GP | G | A | Pts | PIM | |
|---|---|---|---|---|---|---|---|---|---|
| 2009 | Canada | U18 | 4th | 6 | 0 | 0 | 0 | 16 | |
| Junior totals | 6 | 0 | 0 | 0 | 16 | ||||
| Award | Year | |
|---|---|---|
| NHL | ||
| 2x Stanley Cup Champion (Los Angeles Kings) | 2012, 2014 | |