In 1976, while still a teenager, Kinski had her first two major roles: in Wolfgang Petersen's feature film-length episode Reifezeugnis of the German TV crime series Tatort. Next, she appeared in the British horror film To the Devil a Daughter (1976), produced by Hammer Film Productions, which was released in the UK just 40 days after Kinski's fifteenth birthday, making it a virtual certainty she was only fourteen when her scenes were shot (including full frontal nudity). In regards to her early films, Kinski has stated that she felt exploited by the industry. In an interview with W, she said, "If I had had somebody to protect me or if I had felt more secure about myself, I would not have accepted certain things. Nudity things. And inside it was just tearing me apart."
Kinski met the director Roman Polanski at a party in 1976. He urged her to study method acting with Lee Strasberg in the United States and she was offered the title role in Polanski's upcoming film, Tess (1979). In 1978, Kinski underwent extensive preparation for the portrayal of an English peasant girl, which included acquiring a Dorset accent through elocution studies:
In 1976, when Kinski was aged 15, she reportedly began a romantic relationship with director Roman Polanski, who at the time was 43. Polanski confirmed the relationship in a 1994 interview with Diane Sawyer: "...what about Nastassja Kinski? She was young and we had a love affair." However, in a 1999 interview in The Guardian, Kinski was quoted as saying that there was no affair and that, "There was a flirtation. There could have been a seduction, but there was not. He had respect for me."