Martine Kos studied at the Royal Conservatory in The Hague as a vocalist for a couple of years. Even though music is still important in her life, she decided to pursue a career in law. She successfully completed her Master in Dutch law in 2001, after which she started her professional career at Houthoff in Amsterdam. This is where the cooperation between Martine and Hans started.

Hans Londonck Sluijk graduated in 1992 in both Law and Political and Social Cultural studies. He started his career at Houthoff that same year. When Martine joined Houthoff in 2002, Hans had just been named partner and had already made a name for himself as an insurance and litigation specialist. Martine soon followed. The cooperation between Martine and Hans at Houthoff lasted for over a decade. In 2015, Martine decided to accept a position as a partner in a midsize firm in Utrecht. Martine and Hans stayed in touch, occasionally also coming across each other in matters they handled for their respective clients. Eventually, Martine and Hans decided to team up again and start their own boutique firm. As a result, Londonck & Kos was founded on 1 January, 2023.