Lascelles broke into the starting line-up in the 2013–14 season, forming a central defensive partnership with Jack Hobbs and making a total of 29 league appearances. On 22 January 2014, Everton were fined £45,000 by the FA over an "unauthorised approach" to Lascelles in 2010 while he was still a youth player at Nottingham Forest. Lascelles was warned as to his future conduct. On 18 March, Lascelles extended his contract in a new four-and-a-half-year deal. A reported £4 million bid from Queens Park Rangers was rejected by the club in July.
On 9 August 2014, Lascelles signed a contract with Premier League club Newcastle United, joining on the same day as Forest teammate Karl Darlow. As part of the deal, both players were loaned back to Nottingham Forest for the 2014–15 season.
On 27 February 2014, Lascelles received his first under-21 call-up for the 2015 UEFA European Under-21 Championship qualifying match against Wales. He made his debut, and only under-21 appearance, starting in the qualifying match against Moldova on 9 September 2014.