To further develop her craft, Smirnoff participated in a two-year writing course at Lund University. This experience turned out to be a major breakthrough in her professional and literary career. To be accepted into the course, applicants had to submit a sample of their writing. Karin chose to present the first chapter of a book titled Jag for ner till bror (I Went Down to My Brother). In 2018, this work became her debut novel. She later wrote additional installments, forming a trilogy. The novel was a success and received many positive reviews.

Smirnoff’s writing often explores themes of identity, relationships, and the human condition. While she may not be as internationally recognized as some other authors, she has a devoted following in Sweden and across the Nordic countries. Her storytelling, language, and sense of humanity resonate with a wide audience. One particularly unique aspect of her trilogy is the use of dialect—a rare feature in contemporary literature.

In an interview with Boknyheter, when asked why she chose to incorporate dialects, she explained:
"I grew up surrounded by various dialects. My mother is from Storuman, and my father from Överkalix. In Hertsånger, when I was a child, people spoke a rough rural dialect. My partner, who comes from an inland Västerbotten village, speaks his own variation. We live in Piteå, where older people speak Pitemål, an old Swedish dialect that is difficult to understand. Dialects are fascinating. They are more like languages. I wanted them to be heard throughout the story’s setting, even though I use words from both here and northern Sweden."

Overall, Smirnoff’s style is characterized by deep insight, emotional resonance, and meticulous attention to detail, all of which come together to create immersive and thought-provoking narratives.

Additionally, she is the first woman entrusted with continuing Stieg Larsson’s highly popular Millennium series.