Martens began her professional career playing for Dutch, German and Belgian clubs relatively close to her hometown of Bergen, Limburg. In 2015, Martens broke out on the international stage when she scored the Netherlands’ first ever goal in a Women’s World Cup against New Zealand. In 2016, Martens’ performances at Swedish club FC Rosengård attracted the attention of FC Barcelona, who signed her the following year. Weeks after her signing at FC Barcelona was made official, Martens participated in the Netherland’s 2017 UEFA Women’s Euro campaign. She was the Netherlands' standout player at that year’s Euro, where the Dutch won their first ever major international title with the women’s national team. She was chosen as Player of the Tournament, and later in 2017, she was voted as The Best FIFA Women's Player as well as the UEFA Women's Player of the Year.