Early career Letheren was born in
Llanelli, Carmarthenshire. He began his career as a trainee at
Swansea City, spending time on
loan at
Newport County in 2005. He did not make an appearance for the first-team before being released in the summer of 2006. He subsequently joined
Barnsley. Having arrived at Barnsley on a short-term deal, Letheren then signed a new deal to keep him at
Oakwell until 2008 Two years after joining the club, he made his debut in the third round of the
FA Cup against
Blackpool on 5 January 2008, as a half-time substitute, replacing the injured
Heinz Müller. At 1–0 down 'the Tykes' went on to win the game 2–1. He was loaned to
Doncaster Rovers for the second half of the
2008–09 season as cover for veteran goalkeeper
Neil Sullivan, but he did not make an appearance for them. He was released from his contract in the summer of 2009 and then signed a short-term contract with
Plymouth Argyle in the
Championship to provide cover for
Romain Larrieu. On 31 December 2009, Letheren was released from Plymouth Argyle after his contract expired. In April 2010, he agreed to join
Motherwell until the end of the season as an emergency goalkeeper, but the
Scottish Premier League blocked the move as they were deemed to have enough goalkeepers. He remained at
Fir Park to train with the squad for the remainder of the season.
Kilmarnock in 2012 Letheren signed for
Kilmarnock in August 2010, to provide backup to
Cammy Bell. Letheren failed to make an appearance for Kilmarnock in his first season, mostly down to the form of Bell. In 2011–12, he was handed his
Scottish Premier League debut as a half time substitute for the injured Bell, ironically against Motherwell, at 0–0 and Kilmarnock went on to win the game 2–0. Later that year he was handed his full SPL debut away to
Dunfermline Athletic, in a 2–1 win. During the 2012–13 season, Cammy Bell suffered an injury leading to Letheren having a run in the first team. That run lasted until mid-October, when Bell returned. His second run in the first team came in early-February, when he played twice, he was involved in a 2–0 win over motherwell & in what was his last appearance for the club, a 1–1 draw against
Inverness Caledonian Thistle. With his contract due to expire at the end of the season, he left Kilmarnock by mutual consent in April 2013.
Dundee After leaving Kilmarnock, Letheren was linked with a move to South African side
Ajax Cape Town. Letheren went on trial with
Dundee, with Manager
John Brown saying he had done well. After making an appearance in a pre-season friendly, as a trialist, during which he suffered a thigh injury, it was then announced on 9 July 2013, that Letheren had signed a contract with Dundee. Letheren played as first-choice goalkeeper for Dundee throughout all of the 2013–14 season. He was voted supporter's association Player of the Year after pulling off a match-winning save in the dying moment of the final game of the season to secure Dundee the
Scottish Championship title. Letheren played the first ten matches at the start of the 2014–15 season before injuring his knee in a warm up against Motherwell. It was later announced that he would be out until December. After making his return against
Partick Thistle and
St Mirren, Letheren was once again sidelined after suffering an injury in the warm-up ahead of the
Dundee derby against
Dundee United, which allowed
Arvid Schenk to take his place, although Schenk went on to concede six goals, in a 6–2 loss. Though he recovered from his injury, Letheren lost his first choice goalkeeper status to
Scott Bain throughout the season. However, Letheren was given a chance to play the last three matches at the end of the season following Bain's injury. At the end of the 2014–15 season, it was announced that Letheren was being released by the club upon the expiry of his contract. Following his release, Letheren was linked with a move to Championship club
Bristol City. Letheren explained the reason for his release, saying he wanted regular first team football in the hope of securing a place in the national team.
Blackpool and York City Letheren returned to England, agreeing a deal to join
League One club
Blackpool on 1 July 2015, signing a two-year contract, with an option for a further year. Letheren's contract with Blackpool was terminated by mutual consent on 25 August 2016 in order for him to join
National League club
York City. He was signed shortly after York suspended goalkeeper
Scott Flinders. On 21 May 2017, Letheren started as York beat
Macclesfield Town 3–2 at
Wembley Stadium in the
2017 FA Trophy final. He was released at the end of
2016–17.
Plymouth Argyle and Salford City in 2022 Letheren re-signed for League One club Plymouth Argyle on 5 September 2017 on a free transfer. He made his first appearance for Argyle on 26 September, over eight years after he first signed for the club. He was offered a new contract by Plymouth at the end of the 2017–18 season. He was again offered a new contract by Argyle at the end of the 2018–19 season, but turned it down after new manager
Ryan Lowe stated he wanted to bring in a new goalkeeper whether Letheren stayed or not. Letheren signed for newly promoted League Two club
Salford City on 16 July 2019 on a one-year contract. He was released at the end of the
2019–20 season.
Chesterfield On 28 August 2020, Letheren signed for
National League side
Chesterfield. On 24 January 2021, Letheren left the club by mutual consent.
Morecambe On 26 January 2021, Letheren signed for
League Two side
Morecambe. In June 2021, following
Morecambe's promotion to
League One, Letheren signed a one-year deal with the club. On 8 February 2022, Letheren left the Shrimps by mutual consent.
Hartlepool United On 16 June 2022, Letheren signed as player and goalkeeper coach for
League Two side
Hartlepool United. He made his Hartlepool debut in a 6–0 home defeat to
Everton U21s in the
EFL Trophy. In June 2023, he left the club by mutual consent. ==International career==