Alan, from Carryduff, graduated through the junior ranks at Linfield and made his breakthrough into the senior squad in 2002.

In his time at Windsor Park Alan made over 250 appearances for the club and on 25 October 2003 he scored from his own goalmouth against Omagh Town. He won four league titles in his time at the club along with four IFA Cup winners medals.

In August 2009 Alan signed for Michael O'Neill at Shamrock Rovers and was first choice 'keeper at the club for the next two seasons. He won a league title with the club in 2010 but left in July 2011 to join St Johnstone ahead of their 2011/12 Scottish league season.

In April 2018 it was announced that Alan had opted to leave St Johnstone after seven seasons and return to Ireland with Shamrock Rovers days later announcing that they had signed the goalkeeper.

Mannus made over 150 appearances and winning the Scottish Cup in May 2014, and was included in Northern Ireland's squad for the 2016 European Championship.

Mannus left St Johnstone at the end of the 2017/18 Scottish season and returned to Shamrock Rovers for a second spell in green and white hoops.