Kathleen competed in the Australian television talent show New Faces – where she came third in the grand final – and appeared on various Australian television shows, including Heartbreak High, Ridgey Didge, Swap Shop, and the At Home Show with John Mangos.

It was while she was performing at a fundraiser for The McDonald College that de Leon was discovered by one of the directors for the Cameron Macintosh company. De Leon appeared in two musicals in Australia mounted by the Macintosh company – Miss Saigon and Rent – and stayed with the cast of Rent (where she was an understudy for the role of Mimi) until Hi-5 came along.

More information can be found in the Hi-5 article.

In 2007, it was announced that de Leon was leaving Hi-5 permanently to focus on raising her child. She also announced that she would be still be interested in working on television if an opportunity existed in Sydney so she could remain close to her family. Kathleen is the first of the three original members to permanently leave Hi-5, the others being Tim Harding and Charli Delaney.