Themmen was born in Boston to Ivana Marburger (born 1936) and Harold B. Themmen, both classical musicians. Themmen’s mother was a renowned composer for orchestras in the late 20th century. A guitar concerto of hers was in final contention for the 1982 Kennedy Center Friedheim Award. Themmen’s father, a graduate of the New England Conservatory, was a clarinetist and librarian for the American Ballet Theatre, and also played for the Boston Pops.
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. In 1971, aged 11, he got his breakthrough role in Willy Wonka and the Chocolate Factory as Mike Teevee.
Declaring a hiatus from acting at age 14 to “just be a kid,” Themmen went on to receive a B.F.A. in theatre from New York University. He founded Access International, a travel service that arranged Europe-bound charter flights for backpackers. 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.
Themmen’s adult acting appearances include “Virtuoso”, a 2000, sixth-season episode of the TV series Star Trek: Voyager, as a fawning fan. He also was billed as a “former child star” in two 2008 episodes of the American game show Duel. 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 Bollner (Augustus Gloop). They made an additional 40th Anniversary reunion appearance on NBC’s Today Show eleven days later. Paris also appeared on the TV version of Trivial Pursuit in Summer 1993 and won, and was on Win Ben Stein’s Money and lost. On January 23, 2015 Themmen appeared on Ken Reid’s TV Guidance Counselor Podcast. The episode was recorded live in Wilmington, Massachusetts during North East ComicCon. NOn October 28–30, 2015, his wife appeared on Jeopardy! as a contestant, returning twice as a champion before being defeated on her third appearance. Themmen himself was a Jeopardy! contestant on March 13, 2018, finishing in second place.