Mannix is a product of Liverpool Football Club's Academy, and played regularly for the reserves, making over 60 appearances. Mannix was one of the youngest ever players to play for Liverpool reserves: he was 15 and was moved to train at Melwood with the first team aged 16. His progress was restricted by a number of serious injuries, including an injury to his right knee in 2001. He was sent to the world-renowned Dr Richard Steadman in Colorado, America who specialises in knee surgery. Under-18 Academy manager John Owens said: "At Under-15/Under-16 level David was such a shining light in central midfield. He is a very strong but gifted midfield player. But for his injuries he would be playing regularly for the reserves and making tremendous progress towards being the next Academy player through to the first team squad." In November 2006 he went on loan to Accrington Stanley, but after making a promising debut and being selected in the league 2 team of the week, a slight injury picked up in the game forced him to return after less than a week after just one appearance. On 24 January 2007 it was announced that his contract with Liverpool had been terminated by mutual agreement, and on 29 January he signed a three-year contract with the Norwegian club Ham-Kam, who play in the Adeccoligaen having been relegated from the Tippeligaen during the 2006 season. While at Ham-Kam, David helped the side to get promoted back to the Tippeligaen after one season. He returned to England in January 2008, rejoining Accrington Stanley before being released at the end of the season. On 25 June 2008 he joined Chester City on a two-year contract along with Accrington teammate Jay Harris. On 17 October 2008, Mannix was hospitalised with facial injuries following an incident at Chester's training ground. After this incident, Mannix managed to fight his way back into the first team and played in every game for Chester from the end of January. Mannix managed to score his first professional league goal on 28 February 2009 against Dagenham and Redbridge for Chester with a header into the top right corner. Mannix's second league goal for Chester came in their 2–1 victory against Notts County on 4 April. Following on from the end of his ban for betting he went on trial with League 2 club Morecambe and German 3rd division team Unterhaching. In October 2010 he joined Vauxhall Motors. Mannix made an important start to his career at Vauxhall Motors by scoring the only goal of the game in their 1–0 victory against Newcastle Town to help the side reach the FA Cup first round. After a spell with Colwyn Bay, Mannix signed for Warrington Town in January 2013. In June 2015 Mannix signed for Welsh Premier League club Rhyl.
Match fixing/betting scandal
On 7 April 2009 Mannix was charged with breaching the FA's rules on betting. He was accused of placing a bet of approximately £4000 on Accrington Stanley losing against Bury in the final game of the 2007/2008 season. On 22 July 2009 Mannix was given a 10-month ban and fined £4,000 for these betting offences.
Coaching career
On 17 May 2019, Mannix was appointed assistant manager of Clitheroe under newly appointed manager Stuart Mellish.
