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His career at triple j started in 1988 as a producer for station presenters George Wayne, Tracee Hutchison and Tim Ritchie. In 1990, new management at the station forced Kingsmill and many others to re-apply for their jobs. Kingsmill failed to regain his producer position, but landed a presenter position instead.
Since then, he has hosted shows that involve an in-depth knowledge of the current state of the alternative music scene, at both a local and international level. These shows have included The J-Files (1995–2003) and the Australian Music Show (1993–2003).
In 1994, he began hosting triple j's New Releases show. Two years later, this evolved into 1996, a show dedicated to the latest release music from around the world. (The show has changed its name each year, to reflect the current year.) His 2010 show was voted Best Radio Show of the Year by SPA (Street Press Australia) readers nationwide.
In 2003, Kingsmill was promoted to the position of Music Director after the retirement of Arnold Frolows, who had been in the position since Double Jay's inception in 1975. As a result, he cut back his on-air shifts and currently presents only the new music show on a Sunday night.
In May 2012, Kingsmill was voted International Music Director of the year at the Worldwide Radio Summit in Los Angeles.
Kingsmill is the author of two books; triple j's Internet Music Guide (co-written with Stuart Coupe in 1998) and The J-Files Compendium (2002). The latter is a collection of his many music interviews, songlists, quizzes and music trivia sourced from The J-Files.
Kingsmill appeared in the Radiohead documentary Meeting People Is Easy (1998).
In 2009, Kingsmill was executive producer of Before Too Long: Triple J's Tribute to Paul Kelly. The show ran over two nights at the Forum Theatre in Melbourne in November with various artists performing tracks by Paul Kelly, which was recorded for a 3× CD of the same name released in February 2010. It featured John Butler, Missy Higgins, Megan Washington, Dan Sultan, Bob Evans, Paul Dempsey, Katy Steele, Adalita, Clare Bowditch, Dan Kelly, Ozi Batla and Jae Laffer. Along with the show's music director, Ash Naylor, Kingsmill accepted the ARIA Award in 2010 for Best Original Soundtrack/Cast/Show Album.
Kingsmill’s older brother, Mark, is the drummer in Australian rock band, Hoodoo Gurus.