Kathryn Mullen joined the Muppets in 1978, starting with The Muppet Movie, and became the third full-time female puppeteer on The Muppet Show (after Eren Ozker and Louise Gold). She had originally aspired to become an actress and did not join the Puppeteers of America until after a visit to the Muppet Workshop.
Mullen started out with mostly episodic and background characters (notably, Gaffer the cat), as well as assisting on the Muppet feature films. She advanced to a lead puppeteer on The Dark Crystal (operating the heroine Kira) and was one of the core regulars on Fraggle Rock, as Mokey Fraggle and Cotterpin Doozer, among others. She continued to work with the Muppets on a more sporadic basis in the late 80s and 1990s on The Wubbulous World of Dr. Seuss (as a first season regular, notably playing Morton the Elephant Bird, and as a second season director).
One of the creators and performers of Between the Lions, Mullen's other non-Muppet credits included assisting Frank Oz with Yoda in The Empire Strikes Back and the series Allegra's Window where she played the title character. In 2004, Mullen and husband Michael Frith worked with No Strings International to create and perform an educational puppet show for children in Afghanistan, which eventually evolved into The Flying Tent.
- The Muppet Show: The Duchess, Gaffer, Geri and the Atrics (Knitting Lady), Patti Saverne, Japanese Tai-Chi, Karate, and Chowder Society Member, Melissa, Mrs. Appleby, Wanda (1979)
- Fraggle Rock: Mokey Fraggle, Cotterpin Doozer, Crosscut Doozer, Flutebird, Mama Tree Creature (Gorg Bird), Merboo Merggle, Old Gypsy Lady, Rock Hockey Hannah, Wimple Fraggle
- Sesame Street: Grover's Mommy (1981), Mr. Honker (1983), Sally Smith, The Old Woman Who Lives in a Shoe, Lila, Little Miss Muffet ("Little Miss Muffet: The Continuing Story"), Elmo ("Near Far Monsters")
- The Muppet Movie
- The Muppets Go Hollywood
- John Denver & the Muppets: A Christmas Together
- Muppet Meeting Films: Salad in "Meal Break"
- The Great Muppet Caper
- The Muppets Go to the Movies
- The Dark Crystal: Kira (puppeteer)
- The Fantastic Miss Piggy Show
- The Muppets Take Manhattan: Jill
- Follow That Bird: Grouches, various Anything Muppets
- Little Muppet Monsters
- Labyrinth: Goblins (puppeteer)
- The Christmas Toy: Apple
- A Muppet Family Christmas: Mokey
- Free to Be... a Family: Miss Piggy (puppeteer)
- Inner Tube: Babs
- The Muppets Celebrate Jim Henson: Joy Buzzer, Mokey
- Billy Bunny's Animal Songs: Mother Bunny
- The Muppet CD-ROM: Muppets Inside
- The Wubbulous World of Dr. Seuss: Ariana, Aunt Myrtle, Babs Balaban, Backup Singer, Civil Servant #3, Daisy's Mom, Disgusted Guest, Down Whozit, Eliza Jane Dorfman, Finnegan, Gertrude, Hopwood, Junior Kangaroo, Little Cat A, Max (in "The Guest"), Mayor Stovepipe, Molly Livingood, Mom Tidbiddle, Money Whozit, Morton the Elephant Bird, Mrs. Otissa Buttons, Onlooker #1, Pink-Tufted Sneel, Princess Mindy, Queen Myrtle, Queen Regina of Ka-Larry, The Sandwich, Singing Classmate in Play, Squirrel, The Teacher, Tubby Tarbaloot, Verma
- Little Muppet Monsters
- Fraggle Rock animated series
- The Wubbulous World of Dr. Seuss