Growing up in Solihull, England, as an Aston Villa supporter, St Ledger’s career started in the youth system at Peterborough United in 2001, before joining the first team in 2002, playing mostly as a central defender, though occasionally as a full back or in midfield. He joined Football Conference club Stevenage Borough on loan in January 2004. Following a loan spell with Stevenage came to and, St Ledger signed a three-year contract with the club. St Ledger was dropped in the first team and dropped to the bench, citing his bad attitude, in a 1–1 draw with Lincoln City on 21 January 2006.

After a trial with Birmingham City on Barry Fry’s recommendation, St Ledger signed a three-year contract with Preston North End in July 2006 for a fee of £225,000, which could rise to £350,000. Manager Paul Simpsons believed that St Ledger could be a top right-back player.

On 5 August 2006, St Ledger made his debut for Preston in a 0–0 draw with Sheffield Wednesday at centre half alongside fellow new signing Liam Chilvers and since then, St Ledger formed a partnership with Chilvers in central defense. On 9 April 2007, St Ledger scored his first goal for Preston in a 3–2 loss against Southend United. In his first season at Deepdale, St Ledger made 42 appearances whilst scoring one goal as Preston, who had been top of the league mid-season, failed to retain a playoff berth come May.

St Ledger began the 2007–08 season as first choice centre half alongside Youl Mawéné, scoring his second goal for Preston in a 1–0 win over Coventry City on 9 February 2008. The following week, St Ledger received the first red card of his Preston career in a 2–1 defeat at Colchester United, St Ledger taking out United’s Karl Duguid as he went through on goal. St Ledger made 37 appearances for Preston during 2007/08 as the Lilywhites were guided clear of the relegation thanks to a late upturn in results under new manager Alan Irvine. For his impressive season, St Ledger was named the club’s Player of the Year award and had so many awards by the club he have thank them five times.

In September 2009, St Ledger joined Middlesbrough on an initial three-month loan deal with a view to a permanent transfer in the January transfer window, however after failing to settle in the north east, the move was not made permanent and St Ledger returned to Preston.

He made his full debut for Boro on 19 September 2009 against West Bromwich Albion, which Middlesbrough lost 5–0. The next game after making his debut, St Ledger scored his first goal for Middlesbrough against Coventry City in late September. A week later, on 3 October 2009, St Ledger scored his second goal of the season, in a 2–0 win over Reading. He then played against parent club, Preston North End, which described his return as “bittersweet” and was applauded by both set of supporters during the match.

After being called up by newly Manager Giovanni Trapattoni, He made his debut in a friendly against Nigeria at Craven Cottage on 29 May 2009, and played his first competitive match a week later, a 1–1 draw away to Bulgaria in a qualifying game for the 2010 FIFA World Cup. St Ledger scored his first goal for Ireland in a 2–2 with Italy at Croke Park, Dublin, in a FIFA World Cup qualifier in October 2009. The goal gave Ireland a late lead against the World Champions and was almost St Ledger’s “Ray Houghton” moment, guaranteeing him iconic status in only his sixth appearance, only for Alberto Gilardino to score an even later equaliser for the Italians.

St Ledger’s final season at Deepdale came in 2010–11 when he switched shirt from number twelve to four. However, his season was blighted by injury and loss of form as Preston struggled under Ferguson. Despite Ferguson’s dismissal in December 2010 and an eventual improvement in results under successor Phil Brown, St Ledger could not help Preston being relegated to League One. St Ledger played his final game for Preston on 19 March 2011 at home to Coventry before injury finished his season early. In his fifth and final season, St Ledger played 32 times from Preston, scoring his final goal for the club during the campaign at Watford in a 2–2 draw. Considered too good for League One football and with Preston desperate for both money from transfers and from a drop in the wage bill St Ledger left Deepdale in the summer of 2011 to join Leicester City.

On 12 October 2010, St Ledger scored his second goal for Ireland in a UEFA Euro 2012 qualifier against Slovakia which finished 1–1.

On 4 July 2011, St Ledger joined Leicester City on a three-year contract for an undisclosed fee. He made his debut on 17 August 2011 in a 2–1 loss to Bristol City. After a row with manager Nigel Pearson on being unhappy about being dropped for the Boxing Day fixture against Ipswich Town, St Ledger was told he would never play for the Midlands club again and was transfer listed. Leicester and Ipswich Town agreed a fee for the defender but Ipswich manager Paul Jewell said although everything was agreed an injury in training to Leicester defender Aleksander Tunchev put the deal on hold as St Ledger was needed for cover.

On 10 June 2012, he scored his third international goal against Croatia in Ireland’s first group stage game at UEFA Euro 2012. His header was the equaliser to make the score 1–1 after Ireland had fallen behind in the first half, though they lost the game 3–1. It turned out to be the team’s only goal of the competition, as they exited the group stage with three defeats from three games and 9 goals conceded.

On 28 May 2013, St Ledger announced he had been told he was surplus to requirements and had been advised to look for a new club. He made his first appearance of the 2013–14 season in Leicester’s 2–1 opening day win over Middlesbrough and in the League Cup he scored the winner in a 2–1 win over Wycombe Wanderers. These would turn out to be his only appearances of the season as he suffered a recurrence of the knee injury that caused him to miss a large part of the previous season.

St Ledger joined Ipswich Town on a short-term contract on 27 November 2014. However, he made no appearances for Ipswich and his only involvement in the team was being named as an unused substitute for the club’s wins over Charlton Athletic and Leeds United. On 29 December, he was released by the club.

After a successful trial, St Ledger signed with Orlando City of Major League Soccer on 5 March 2015. His contract was terminated by the club on 1 August 2015 after a “serious breach of club policy”. The Orlando Sentinel reported that Orlando City SC released St Ledger for missing a team flight and several days of training.

Whilst recovering from long-term injury after leaving Orlando, St. Ledger scouted for former club Leicester City. He played for the club’s under-21 side in a pre-season friendly against Leamington in July 2017.

In October 2017, St Ledger joined Solihull Moors of the National League, making his debut in a 0–2 defeat to Wycombe Wanderers on 5 November. He did not make a league appearance for the club and, in January 2018, joined Solihull’s divisional rivals Guiseley, where he took caretaker charge of the team between February and May 2018.

As of 2019, St Ledger was working as senior scout at Leicester City.