His career started with a small appearance in the 2002 film Hilfe, ich bin ein Junge (English: Help, I’m a Boy!). In December 2003, he joined Blaues Wölkchen, a small group from a children’s theatre in Bargteheide. His first major theatrical appearance was in Hilfe, die Herdmanns kommen.

In 2005, Kross came to the attention of Detlev Buck through his daughter, Bernadette, and he auditioned for Tough Enough (Knallhart); Buck hired Kross to play the lead, a 15-year-old boy who moves with his mother from a rich neighborhood of Berlin (Zehlendorf) to the Neukölln area, known at the time for its high number of Turkish immigrants and high crime level. Kross not only won praise at Berlinale in 2006, but also won Best Actor in Nuremberg at the 11th Filmfestival Türkei/Deutschland.

In 2006, Kross worked again with Buck in the film Hands off Mississippi [de] (Hände weg von Mississippi), playing an apprentice baker. In the fall of the same year, he started shooting a film by Marco Kreuzpaintner, Krabat (The Satanic Mill). In this version of Otfried Preußler’s children’s book, Kross plays the title role, one of the apprentices of magic, with co-stars Daniel Brühl and Robert Stadlober. The film was released on September 19, 2007 in movie festivals and in October 2008 in theatres.

In September 2007, The Reader began shooting in Berlin, Cologne, and Görlitz. In Stephen Daldry’s adaption of the best-selling novel by Bernhard Schlink, Kross plays the lead role of Michael Berg, opposite Kate Winslet, Ralph Fiennes, and Bruno Ganz. He had to learn to speak English to appear in the film. The world premiere was at the Ziegfeld Theatre in New York on December 3, 2008. The film was presented in the 2009 Berlinale but did not compete. In May 2009, Kross was honored for his performance in The Reader at the 62nd Cannes Festival, winning the Chopard trophy. Kross was nominated for a European Film Award as best actor.

In 2009, Kross started a three-year course of studies at London Academy of Music and Dramatic Art (LAMDA). His plan was to improve his acting skills and his English. However, he dropped out at the end of the same year to concentrate on films. Since then, he has been living in Berlin-Mitte. Kross has expressed no desire to move to Hollywood, preferring to reside in Germany and continue making both German and English language films.

In June 2010, it was announced that Kross had been cast in Steven Spielberg’s film, War Horse. Filming started in August 2010, in Dartmoor, Devon, U.K. and the film was released in December 2011.