From 1998 to 2009 Aerts was a member of the successful girl group K3. Originally thought as the Flemish version of the Spice Girls, the group eventually became famous for their songs for children. Because K3 was originally envisioned as the Flemish version of the Spice Girls, the group was made up of all females in their twenties: Aerts, Karen Damen, and Kristel Verbeke.

K3 achieved success after the release of "Heyah Mama", and in 2002 their manager sold the band to Studio 100. The production company made the group an even bigger success, with merchandising, film, TV series and books also being released.

In 2009 Aerts decided to stop being a member of K3 and to start her solo career. Her first song, titled "Zumba Yade", was made in cooperation with Unicef. The single reached the top 3 of the Dutch Single Top 100 in July 2009 and the top 20 of the Ultratop 50 in Flanders. She sold more than 10,000 CDs in Belgium and the Netherlands. In November 2009, Aerts released her debut solo album Kathleen in symfonie. The album contained twelve songs, including a duet with Danny de Munk. Aerts ensured that part of the proceedings from the album went to Unicef. This decision ensued from her experience in South Africa.

In 2010 she became a member of the KetnetBand, The same year she also decided to go back to children's music, releasing a song titled "Boerderij blues" (en. Farm blues). She released three other songs for children. In 2007 a wax figure of Kathleen Aerts was unveiled in the Madame Tussauds in Amsterdam together with those of the other two original K3 members.

In 2016, Aerts released "Die roos", an adaptation of Ann Christy's hit "De roos", her first single in Afrikaans. A year later she recorded the single "Afrika", also in Afrikaans, which was also part of the soundtrack of the film F.C. De Kampioenen 3: Forever (2007). In September 2017, she published her third book: Lilly en Max in de wereld van oma.

In 2020, Aerts participated as Mermaid in the television show The Masked Singer, in which judges must guess the identity of the masked participant. Here she was reunited after a long time with Karen Damen, who sat on the jury of the program and correctly guessed the masked mermaid was Aerts.

In 2023 has been a website to you with The Kathleen Aerts Company, in the television show with Morph, Big & Betsy and Alfred Jodocus Kwak of the animation test as amazing songs of the company with you!