He was selected by the Los Angeles Kings in the second round (35th overall) of the 2009 NHL entry draft, and spent the majority of his career with the team, though also made appearances with the Toronto Maple Leafs and St. Louis Blues.

[5] After the 2009–10 season, Clifford joined the Kings' American Hockey League (AHL) affiliate, Manchester Monarchs for the postseason.

[6] In 2010, Clifford was among those invited to the Canada men's junior team training camp[7] preparing for the 2010 championship.

[16] Clifford (alongside Tyler Toffoli) assisted on the championship-winning goal scored by Alec Martinez in double-overtime.

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.

[21] As a free agent from the Maple Leafs, Clifford left to sign a two-year, $2 million contract with the St. Louis Blues on October 11, 2020.

[23] Clifford cleared waivers the following day, and a few hours later was traded to the Toronto Maple Leafs in exchange for future considerations.

[26] Clifford played in Game 1 of the 2022 Stanley Cup playoffs for the Maple Leafs against the Tampa Bay Lightning.

Having concluded his NHL contract with the Maple Leafs, Clifford opted to continue his tenure with the Marlies by signing as a free agent to a one-year AHL deal for the 2024–25 season on July 31, 2024.