Alexander Ludwig first appeared in the 2000 direct-to-video comedy film ‘Air Bud: World Pup,’ where he made an uncredited cameo appearance. That same year, he also appeared in the feature film ‘MVP: Most Valuable Primate.’ In 2005, he made his voice debut in the animated film ‘Scary Godmother: The Revenge of Jimmy.’ After making minor appearances in films like ‘A Little Thing Called Murder’ (2006) and ‘The Sandlot: Heading Home’ (2007), Ludwig got his first lead role in the 2007 family drama-fantasy film ‘The Seeker.’ The film was based on the second book of the five-book children's fantasy series ‘The Dark Is Rising’ by Susan Cooper. ‘The Seeker’ was heavily criticized for disregarding the source material. The critics also called the plot “clumsy” and “lacking a heart.”
In 2009, he starred as ‘Seth’ in the American science fiction adventure thriller film ‘Race to Witch Mountain’ directed by Andy Fickman. The film was adapted from the 1968 novel of the same name by Alexander Key. ‘Race to Witch Mountain’ received mixed reviews from critics and was a financial success. In the 2012 dystopian action film ‘The Hunger Games,’ Ludwig played the role of ‘Cato.’ The film received critical acclaim upon release and was a massive box office success, grossing over $694.4 million on a budget of $78 million. He next acted in ‘Grown Ups 2’ (2013), ‘Lone Survivor’ (2013), and ‘When the Game Stands Tall’ (2014). In 2014, Ludwig joined the main cast of the historical drama television series ‘Vikings,’ created by Michael Hirst. He played the role of ‘Björn Ironside’ in the series which earned him a ‘Golden Maple Award’ nomination. The show ran for almost 8 years and aired its final episode in December 2020.
Ludwig has also acted in ‘Final Girl’ (2015), ‘Midway’ (2019), ‘Bad Boys for Life’ (2020), ‘Night Teeth’ (2021), and ‘National Champions’ (2021). In 2021, he starred as ‘Ace Spade’ in the American drama television series ‘Heels.’ The show concluded its first season in October 2021.