In the following years, Ullmann starred in several minor roles in TV series such as Stahlnetz, Großstadtrevier, and SOKO Leipzig as well as in TV movies such as the drama Albtraum einer Ehe (2001) alongside August Zirner. From 1999 to 2000, he was a cast member of the comedy series Zwei Männer am Herd with Wolfgang Fierek and Florian Fitz.
In 2004, Kostja Ullmann made his movie debut in a leading role in Marco Kreuzpaintner's award-winning film Sommersturm (Summer Storm). His performance in the romantic TV drama Heimliche Liebe about the relationship of a schoolboy and an older woman won Ullmann the Günther-Strack-Fernsehpreis as Best Actor in 2006.
In his second movie, Verfolgt (2006), Ullmann stars in a similar role. Directed by Angelina Maccarone, he plays a young delinquent who engages in an "amour fou" with his probation officer who is more than 30 years older than he is. The film won a Golden Leopard at the 2006 Locarno International Film Festival. He acted as Hermann Lake in German-Japan collaborated movie Baruto no Gakuen (Ode an die Freude) in 2006.
Ullmann played the lead role, Luca Behrendt, in the 2008 movie Die Zeit, die man Leben nennt.
