Soon after his club debut he suffered severe burns from a cooking accident and missed almost two months before returning to the line-up on 9 May against FC Edmonton when he scored his first goal for the Whitecaps in the 93rd minute.

[7] 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,[8] but he lost his starting role after Wayne Rooney arrived in the mid-summer as a Designated Player.

[13] Mattocks made his international debut for Jamaica in 2012, coming on in the 55th minute in a 2–0 win over El Salvador.

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.