Internationally, he earned 61 caps for Germany, starring in their 2006 World Cup quarterfinal shootout win against Argentina and reaching the Euro 2008 final.
He joined Arsenal (2003–2008, 2011), becoming a key figure in their "Invincibles" season (2003–04) and playing in the 2006 Champions League final, though he was sent off early. He finished his club career at VfB Stuttgart (2008–2010) before briefly returning to Arsenal in 2011.
In 1993 Thierry joined LOSC Lille where he made over 70 appearances. In 1995 he joined Swiss side Neuchâtel Xamax FCS where he played for 2 seasons. Thierry joined Nottingham Forrest in 1997 after their relegation from Premier League.
He reclaimed his position during Schalke’s 2021–22 promotion campaign to the Bundesliga and continued to provide experience and leadership despite injuries and competition. While he never earned a senior cap for Germany, he remains one of Schalke's most notable goalkeepers.
In January 2023, he returned to Pau FC on loan for the remainder of the 2022–2023 season, adding 18 Ligue 2 appearances and one goal to his record. In 2024, Boisgard joined Apollon Limassol, a club in Cyprus, where he has since made 16 appearances.
Following Toulouse's relegation and Lorient's promotion to Ligue 1, Boisgard transferred to FC Lorient in July 2020 on a four-year contract, with a reported transfer fee of €2 million. Over three seasons, he made 54 league appearances and scored four goals
later became a key player for Birmingham City, where he spent most of his career. After brief spells at Leeds United and Millwall, he retired and transitioned into coaching, working with Birmingham City and the Northern Ireland national team.