[9] In the 2001–02 season, Mannus appeared as an unused substitute in the first qualifying round of the UEFA Champions League against Torpedo Kutaisi, as the club were eliminated from the tournament after losing 1–0 aggregate.
[13][16] After the match, he said: "This time last year I wasn't sure I would be kept on for this season, a month ago I was playing in the Swifts and now I've got an Irish Cup winners medal, it is unbelievable, a really brilliant feeling.
"[17] A few days later, Mannus helped Linfield Swifts win the Belfast Telegraph Intermediate Cup final to beat Donegal Celtic 1–0 to earn himself a double this season.
[49] In a match against title contender Glentoran on 23 April 2005, he started on 150th appearance for Linfield, and despite playing through injury in a collision with his skipper Noel Bailie midway through the first half, the club went on to lose 3–2.
[51] Despite losing the league title, Mannus was in goal for the Setana Cup final against Shelbourne when Linfield won 2–0 to win the inaugural tournament.
[53] At the start of the 2005–06 season, Mannus’ performance against Rangers in a friendly match was praised by both managers, David Jeffrey and Alex McLeish, with suggestions that he should turn professional.
[68] Following the club’s elimination from the tournament, he made a warning to the Northern Irish teams that Linfield’s form was blistering prior to the first league match of the season.
"[73] He made his 250th appearance for the club before his 25th birthday, coming against Drogheda United on 26 February 2007, which resulted in a 4–0 win in the group stage of Setanta Sports Cup.
[100] In July 2009, Mannus was given a trial with English League Two side Bradford City, but after playing three games without conceding a goal, he was released and returned home.
[102] Having spent two weeks becoming the second-choice goalkeeper behind Barry Murphy, he made his debut for Michael O'Neill's side in a 2–2 draw against Dundalk on 22 August 2009.
[110] In a match against St Patrick's Athletic on 5 July 2010, Mannus saved a penalty at the last minute of the game from Ryan Guy to keep the result in Linfield’s favour, winning 2–1.
[138] At the start of the 2012–13 season, Mannus played in both legs of the UEFA Europa League second qualifying round against Turkish side Eskişehirspor, as St Johnstone went on to lose 4–1 on aggregate and was eliminated in the tournament.
[140] In a match against Inverness Caledonian Thistle on 27 October 2012, Mannus saved a penalty from Andrew Shinnie before he set up the equalising goal for David Robertson, in a 1–1 draw.
[142] During a 2–0 loss against Aberdeen on 22 December 2012, Mannus produced " four or five great saves", which local newspaper, The Courier stated that "if it hadn’t been for his heroics in goal, the scoreline could have been a lot worse than 2–0.
[162] After the match, he said about his happiness of returning from injury, but stated: "I have a bit of metal that goes along my finger and I tape it and I have a spine as well that goes in the glove which holds my thumb in position.
At the start of the 2014–15 season, Mannus played in both legs of the UEFA Europa League second qualifying round against Luzern, where the match eventually led to penalty shootout after a 2–2 draw on aggregate.
[181][182] He started in the first leg of first qualifying round of the UEFA Europa League against FC Alashkert and conceded the only goal of the game, as St Johnstone lose 1–0.
[186] In a match against Dundee United on 26 September 2015, Mannus received a straight red card in the 23rd minute for bring down Billy Mckay, as the Saints won 2–1.
[199] However, he suffered an illness that led to him place on the substitute bench, with Zander Clark took over his duties as the club’s first choice goalkeeper role for most of the season.
[215] After being dropped to the substitute bench in late–April, he made his last appearance for the club as captain against Ross County on the last game of the season, as the match ended in a 1–1 draw.
[218] In Mannus's first season back, he would go on to play every minute of their league campaign, having dispatched Gavin Bazunu as Shamrock Rovers’ first choice goalkeeper role.
[230] Mannus started in the final against Dundalk, as he played throughout 120 minutes following a 1–1 draw and helped Shamrock Rovers win on penalties, providing crucial saves in the shootout to lift the FAI Cup.
[241] Mannus also made the crucial save in the club’s epic 13–12 penalty shoot-out win over Ilves in the first qualifying round of UEFA Europa League.
[268] In a follow–up match against Dundalk, Mannus kept another clean sheet and made a number of vital saves, including a second half free-kick from Keith Ward, as Shamrock Rovers won 3–0.
[283] It wasn’t until on 27 July 2023 when he made his return from injury, starting the match, in a 4–0 loss against Ferencvárosi in the second qualifying round of the UEFA Europa Conference League.
[295] In a December 2015 interview, Mannus revealed that prior to becoming cap tied to Northern Ireland no one from the Canadian FA had ever been in touch with him regarding representing Canada.
[75][300] It wasn’t until on 6 February 2008 when he made his first appearance for Northern Ireland in four years in a friendly match against Bulgaria, coming on as a second–half substitute, as the national side lose 1–0.
[301] A month later, on 25 March 2008, Mannus made another appearance for Northern Ireland, coming on as a 80th minute substitute for Maik Taylor, in a 4–1 win in another friendly match against Georgia.
[304] However, Mannus spent four years as a second–choice for the national side and was placed on the substitute bench, due to being overshadowed by Maik Taylor, Lee Camp and Roy Carroll.
[322] Born in Mississauga, Ontario, Mannus lived in Canada until his parents moved back to Northern Ireland, where he was raised in Carryduff in the north of County Down.