Released by Aston Villa at the age of 16, Karl was soon snapped up by Nottingham Forest and went on to make his debut against Crystal Palace on the final day of the 2010/11 Championship campaign.
Spells then followed on loan with Conference side Newport County and League One Walsall, before establishing himself at the City Ground midway through the 2012/13 season.
He maintained his place for the following Championship campaign and his impressive form clinched a move to Premier League side Newcastle United in the summer of 2014.
He was loaned back to the Reds before a permanent move to Tyneside ahead of the 2015/16 campaign, and he made his top-flight debut that December against West Bromwich Albion.
Following relegation at the end of the season, Karl became first-choice keeper in the Championship as the club made an immediate return to the top flight under Rafa Benitez, and he has now made over 60 appearances for the Magpies.