Born in the city of Leipzig which lies in the part of Germany that was once the German Democratic Republic, Kassin is the eldest of three children. She was 10 years old when the Berlin Wall came down and Germany reunited. When she was 15 years old she had her first job selling ice cream at an Italian restaurant after school and on the weekends. She was a waitress for six years during her high school and college time. In September 2000 she started working as a nude model posing for local amateur photographers and websites. After three years, she moved into softcore shoots and eventually came in contact with German porn star Dru Berrymore. Kassin was introduced to Mark Spiegler of the talent agency Spiegler Girls for whom she worked for three and a half years. On September 9, 2006, she signed with LA Direct Models.

Before moving to the United States Kassin studied Political Science at the University of Leipzig in Germany. She has stated that she earned a degree equivalent to an Associate's degree. She also studied German Language and Literature.

Katja Kassin at the Erotica L.A. convention, June 24, 2007, post breast augmentation.

Her first film scene in the United States was "Straight to the A 4" by Vince Voyeur for Red Light District. This was filmed in March 2003; before that she had only done two scenes in Europe. She permanently moved to the Los Angeles area in late-2004 and married a fellow male performer in January 2004.

On June 1, 2007 she got breast augmentation surgery from Dr. Jacobs in New York City. The 350cc silicone implants were inserted through the nipple and under the muscle. Her first shoot after recovery was on June 26, 2007.

Kassin's specialty is rough anal sex, including double anal, though she has since ceased filming creampies after the HIV outbreak in 2004. In Germany Katja is known for her appearances at the German erotic TV show Wa(h)re Liebe.

English, German, Russian and three others. She adopted the first name "Katja" as her pseudonym at the suggestion of the photographer at her first nude shoot, after his niece