Though he never made a Premier League appearance for Arsenal, Stack broke through in cup competitions. His League Cup debut arrived in 2003 when he played in a memorable shootout win against Rotherham United — saving a penalty and converting one himself in a 9-8 victory. He continued to feature in Arsenal’s League Cup run to the semi-finals during the 2003/04 campaign, acting as understudy to Jens Lehmann throughout the Invincibles season, warming up Lehmann before each game and providing back-up without being selected for league fixtures.

In search of regular first-team football, Stack embarked on several loans. He spent the 2002/03 season with Belgian side Beveren (24 appearances), then featured for Millwall (2004/05, 26 games), before a short spell at Reading (2005/06), where he made one league appearance before joining them permanently. At Reading he was part of a promotion-winning squad to the Premier League but sought game time elsewhere.

In October 2006, Stack joined Leeds United on loan from Reading for the remainder of the Championship season. Leeds needed experienced goalkeeping cover amid an unsettled campaign. Initially, Tony Warner and Neil Sullivan were on the books, but performance issues and injury opened the door to Stack’s arrival.

Stack had worked with Leeds manager Dennis Wise at Millwall, and within hours of his appointment at Elland Road, Wise felt the need for men he could rely on and trust. The knock on Stack's door was quick in coming. After three games, two defeats and the concession of seven goals, the 25-year-old could have been forgiven for asking whether the compliment was partially back-handed. His transfer to Yorkshire lodged Stack behind the least reliable defence in the Championship.

Both Wise and Stack knew their side of the bargain, and a quick and brutal introduction to life at United failed to dampen Stack's spirit.

'I looked at the league table before I came here, and I was surprised, said Stack. 'Anyone who follows football and sees Leeds in the bottom three will wonder what's happening. But it didn't deter me from coming here. It's a massive challenge playing for Leeds, and when they're down at the bottom the challenge is even bigger. There were other opportunities, but the offer from Leeds was too good to turn down. The decision took a split-second to make.

'If you put a run together you can jump up the table, and people go from talking about relegation to talking about the play-offs. But we know we're in a dogfight, there's no two ways about it. As big as Leeds United Football Club is, it doesn't mean you have a divine right to get out of the bottom three. Leeds are second from bottom because they deserve to be.'

Stack could not have hoped for a better debut last month, appearing for the first time in Leeds' 2-0 win over Southend. The victory was United's first for five games, and their first clean sheet since August, and both Warner and Sullivan were playing second fiddle to Wise's first-hoice keeper.

He made 12 league appearances for Leeds, keeping the gloves between October and early 2007 before the club brought Casper Ankergren on loan, after which Stack was displaced and ended his spell at Elland Road.

Stack's time at Leeds coincided with instability both on and off the field. Manager Dennis Wise was experimenting with signings through the loan market to try to arrest a slide down the table. Stack added solidity and reliability between the posts during a difficult period. Although he wasn’t able to establish himself beyond the 12 appearances, his contributions included several clean sheets and solid shot-stopping under pressure.

The arrival of Danish goalkeeper Casper Ankergren in January 2007 shifted the pecking order. Ankergren quickly impressed fans and coaches, regaining the No.1 spot and steaming ahead in form. Stack was subsequently sidelined and returned to his parent club once Leeds had signed Ankergren for the remainder of the season. Despite Stack’s earlier stint holding the shirt, the Danish loan signing won the support of supporters rapidly.

Following the end of the loan, Stack continued to move around on short-term deals: a further loan at Wolverhampton Wanderers yielded only two appearances before he departed Reading in 2008. He then had stints with Plymouth Argyle (5 appearances), another unfruitful loan at Blackpool, and then a return on loan to Wolves where he made no first-team appearances. In July 2009, he signed for Hibernian in the Scottish Premier League, where he made 56 league appearances across three seasons and played in the Scottish Cup final, as well as UEFA Cup matches.

In 2012 he joined Barnet, eventually making 125 league appearances and captaining the side. He helped lead Barnet to the Conference Premier title in 2014/15 and was named in the PFA Community Champion list in 2015/16. During this period, he also took on first-team coaching responsibilities, gaining his earliest experience of dual roles as player and coach.

After leaving Barnet in 2016, Stack moved to India to play and coach at Kerala Blasters in the Indian Super League. During his four months in India, he featured in front of crowds of 60,000–70,000 and played in the ISL final, finishing runners-up. Off the pitch, Stack embraced the cultural experience and used the time productively, studying for coaching qualifications and learning to work in a vastly different football environment.

In January 2017 he joined Eastleigh as player-goalkeeper coach, making 51 appearances before retiring from playing in 2018. Following his retirement, he took up a series of coaching positions: head of academy goalkeeping at Watford (2018–2022), first-team goalkeeping coach at Cardiff City (June 2022–Feb 2023), and then at Colchester United. In July 2023, he briefly joined Maccabi Tel Aviv as first-team goalkeeping coach under Robbie Keane before departing that role the same month.

Internationally, Stack represented the Republic of Ireland at U21 level seven times and earned guidance from senior call-ups, though he never gained a full senior cap.