Born in Dublin, Karl began his career at his local side St Kevins Boys. At the age of 16 he decided to cross the water and join Manchester City after the Eastlands outfit offered him a deal following a successful trial. Moore was part of the Manchester City youth side beaten 3–2 by Liverpool in the final of the FA Youth Cup in 2006. On 29 August 2008 he went out on loan from Manchester City to Millwall until January 2009 and made his début for the London club the very next day in Millwall's 2–1 home win against Huddersfield Town. He went on to make a total of nine appearances for the Lions during his loan spell there. Moore plays primarily as a left/right winger but has been known to be capable at filling in at various other positions. He has represented his country at every level from 'Under 15' up to and including 'Under 21'. On 8 June 2010, Karl was released along with Martin Petrov, Sylvinho, Jack Redshaw and Benjani. In August that year, Moore returned home to join University College Dublin AFC in a bid to regain fitness and match sharpness as he stepped up his return from injury. Karl made his League of Ireland debut for UCD AFC in a game against Sligo Rovers as a late substitute and soon became a regular with the students. Karl left UCD AFC at the end of the 2010 season, despite being asked to stay on. In February 2011 Karl signed a one-year deal with Connacht outfit Galway United, although there was also interest from several other League of Ireland clubs. It was rumoured, on 12 June 2011, that Moore may make a return to British football, with SPL club Motherwell understood to be leading the chase for his signature. However, on 3 August, it was announced that Karl had signed for Airtricity League Champions, Shamrock Rovers, after leaving Galway United due to financial troubles at the club . Karl joined Rovers' fierce rivals Bohemians on 13 January 2012, and scored on his competitive debut against Portadown FC in the Setanta Cup. His first league goals for the club came on 29 June when he netted a brace in a 4–0 thumping of Shamrock Rovers at Dalymount Park. Despite interest from other clubs, Moore signed a new one-year contract with Bohs at the end of the 2012 season.
