A former youth keeper at Aston Villa, Darlow joined Nottingham Forest in 2006 upon his release to sign up for the club’s Academy.
He soon impressed and following a run in the Reds’ reserve side he was promoted to the first-team squad for the 2011-12 season, where he was present on the bench as cover for Lee Camp.
Karl was handed his debut on the final game of the season against Crystal Palace, as substitute for Camp, and kept a clean sheet in a 3-0 win.
In March 2012 he joined Conference side Newport County on loan while in September of that year he spent time at Walsall. Darlow re-joined the Saddlers in January 2013 but his spell was cut short when Camp left for Norwich – and he made his Forest debut that month against Peterborough United. He then kept his place in Billy Davies’s side, and has earned praise for his assured performances.
Following Davies' departure, Darlow maintained his place in the Forest side and in August 2014 he signed a permanent contract with Premier League Newcastle United. He was immediately loaned back to the City Ground club for the 2014/15 campaign.
Karl became an integral part of the Newcastle squad for 2015-16 and although the Geordies were relegated, they lost just three of the nine Premier League games in which he played towards the end of the season.
He maintained his position under Rafa Benitez in 2016-17 and played a key role as the Toon made an instant return to the top-flight, winning the Championship trophy ahead of Brighton on the final day of the season.