After being invited for a training ground tour, Mignolet signed a five-year contract with English Premier League club Sunderland on 17 June 2010 in a deal worth £2 million. It was later confirmed by Sint-Truiden that Sunderland had beaten PSV, Twente and Udinese to secure Mignolet's signature.
Mignolet made his Sunderland debut in a 2–2 draw against Birmingham City on 15 August 2010, and was named man of the match for his performance. In the next home match, a 1–0 victory against Manchester City, Mignolet kept his first clean sheet for Sunderland, following an excellent reaction save to deny Emmanuel Adebayor a goal. Mignolet put in another fine performance against Arsenal, as Sunderland drew 1–1 after Darren Bent equalised in the 95th minute. However, Craig Gordon would replace Mignolet in goal several weeks later, and remained first-choice thereafter. Mignolet did not feature again until 8 January 2011, when Sunderland suffered a loss at home to Notts County in the third round of the FA Cup. Mignolet would become first-choice goalkeeper again after a knee injury to Gordon in February, and earned a string of rave reviews for his performances. Following the draw, Sunderland manager Steve Bruce hailed the young goalkeeper's fine start to life in English football. Mignolet cemented his position as first-choice goalkeeper the following season, being preferred to new signing Keiren Westwood.