In January 2011, he joined Sheffield United on loan until end of the 2010–11 Football League Championship season making his first start for "the Blades" away at Coventry City a couple of days later. He went on to make 13 appearances for the Yorkshire club but was unable to help prevent them from being relegated at the end of the season. In November of the same year, Mattock joined Championship team Portsmouth on a loan deal lasting until 2 January 2012, alongside George Thorne. He made his debut two days later, starting in the 1–1 draw against Leicester City.