Mattocks was selected by Vancouver Whitecaps FC with the second overall pick in the 2012 MLS SuperDraft.

Mattocks scored his first MLS goal on 26 May 2012 against the Portland Timbers. Mattocks scored six goals in his first nine MLS appearances in 2012. In the Whitecaps’ first ever playoff game versus the LA Galaxy, Mattocks scored the first ever playoff goal for the Vancouver Whitecaps in the fourth minute of play. He earned the team’s Golden Boot award that year with seven goals.

Mattocks made his international debut for Jamaica in 2012, coming on in the 55th minute in a 2–0 win over El Salvador. He scored three times during the 2014 Caribbean Cup, making him the tournament’s joint top scorer, as Jamaica won the tournament for the sixth time.

Mattocks received his U.S. green card in May 2014, which qualifies him as a domestic player for MLS roster purposes.

In 2015, Mattocks was included in Jamaica’s squads for the Copa América and CONCACAF Gold Cup tournaments. On 22 July 2015, Mattocks scored the opening goal in the 2–1 semi-final victory over the United States in Atlanta, Georgia, as the Reggae Boyz qualified for their first ever Gold Cup final. He also scored in the 3–1 defeat to Mexico in the final in Philadelphia.

Mattocks was traded in March 2016 to Portland Timbers in exchange for targeted allocation money and general allocation money.

On 10 December 2017, Mattocks, after having his contract option rejected by Portland, was traded to D.C. United in exchange for an international roster slot. D.C. United exercised Mattocks’ option for 2018, but he lost his starting role after Wayne Rooney arrived in the mid-summer as a Designated Player.

On 11 March 2018, Mattocks scored his first MLS regular season goal for D.C. United. He scored the only goal for D.C. United in the 3–1 loss against Atlanta United. In his time with DC United, he appeared in 25 games and scored 10 goals.

In December 2018, Mattocks was drafted by FC Cincinnati in the 2018 MLS Expansion Draft.

In February 2020, Cincinnati head coach Ron Jans resigned following allegations of racist behaviour after the MLS Players Association received a complaint from a player. The MLSPA did not publicly identify the complainant, but some Dutch media outlets speculated that it was Mattocks.