He entered the stage at the age of eleven and performed at Hamburg"s Ernst-Deutsch-Theater and Winterhuder Fährhaus. In 1998, he made his television debut in a minor part in the crime series Alphateam. In the following years, Ullmann starred in several minor roles in television series such as Stahlnetz, Großstadtrevier, and SOKO Leipzig as well as in television movies such as the drama Albtraum einer Ehe (2001) alongside August Zirner.
In 2004, Kostja Ullmann made his movie debut in a leading role in Marco Kreuzpaintner"s award-winning film Sommersturm (Summer Storm).
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.
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.
Ullmann will voice Wirt in the German dub of Over the Garden Wall.
From 1999 to 2000, he was a cast member of the comedy series Zwei Männer am Herd with Wolfgang Fierek and Florian Fitz.
