Jennings started his career at Tottenham Hotspur before joining UCD in January 2001 on a Sports Management Soccer Scholarship. He moved north to Derry City after several seasons at Belfield Park. He unsuccessfully tried to displace first choice keeper David Forde, but rejected offers to leave the club at the end of the 2005 season. Jennings was able to be the hero for Derry in the 2006 League of Ireland Cup final against Shelbourne as he came on from the substitute's bench to fill the place of Forde, who had been sent off in the 84th minute of play. The game went to penalties and Jennings saved all of the Shelbourne's attempts. With the departure of Forde to Cardiff City at the end of the 2006 season, Jennings began the 2007 season as Derry coach Pat Fenlon's first-choice goalkeeper despite the signing of Ola Tidman. Pat joined Sligo on loan for the 2008 season. Jennings signed for Shamrock Rovers in July 2010 but never played competitively. In March 2011 he signed for Glenavon until the end of the 2010/11 season. On his debut he scored an own goal. In March 2012 he joined Athlone Town. Jennings signed for St Patrick's Athletic as a goalkeeper coach to goalkeepers Brendan Clarke and Dave Ryan for the 2013 season. Ryan picked up an injury towards the end of pre-season and Jennings was signed up as the club's third choice goalkeeper on 6 March. With Pats' sub keeper Rene Gilmartin picking up an injury in a Setanta Cup game, Jennings was once again signed up as cover to Brendan Clarke for the 2014 season. With Gilmartin out injured, manager Liam Buckley decided not to risk injury to Clarke for the Leinster Senior Cup quarter-final away to Athlone Town and gave Jennings his debut as Pats won 3–2 after extra time against Jennings' old club. Jennings played in the 2014 Leinster Senior Cup Final on 9 September 2014, as Pats beat Longford Town 2–1 to become champions. His next appearance came on 22 September2015 in the 2015 Leinster Senior Cup Semi-final vs. Dundalk. With first choice keeper Brendan Clarke out for the season with a broken finger, manager Liam Buckley opted not to risk young stand in keeper Conor O'Malley in the competition of least importance to the club and played Jennings, 2 days before his 36th birthday as Pats lost 1–0 with Jennings saving a John Mountney penalty in the 59th minute.