[5] Themmen's father, a graduate of the New England Conservatory of Music, was a clarinetist and librarian for the American Ballet Theatre, and also played for the Boston Pops Orchestra.

[6] Themmen appeared in radio and TV commercials, voice-overs and theater performing on Broadway in Mame with Ann Miller in 1967 and in The Rothschilds in 1970.

Following brief stints in real estate, film production, commercial casting, business representation at Walt Disney Imagineering, and a few other ventures, he now signs autographs at movie conventions, runs a photography business, and makes sporadic appearances in commercials, plays and TV shows.

[7] Themmen's adult acting appearances include "Virtuoso", a 2000, sixth-season episode of the TV series Star Trek: Voyager,[8] as a fawning fan.

[9][10] On May 4, 2011, Themmen appeared on the British television morning show Daybreak, alongside the other child actors from Willy Wonka & the Chocolate Factory: Peter Ostrum (Charlie Bucket), Julie Dawn Cole (Veruca Salt), Denise Nickerson (Violet Beauregarde) and Michael Böllner (Augustus Gloop).