Mullen began performing on The Muppet Show in its third season, primarily as Gaffer the Backstage Cat.

She also voice directed Dog City for Nelvana, worked as a designer for The Muppet Movie, right-handed for Frank Oz on Yoda on The Empire Strikes Back,[2] and was one of the creators of Between the Lions.

Mullen, with her husband, former Muppet designer Michael K. Frith, founded "No Strings" along with emergency aid worker Johnie McGlade.

The company originally created a film for children in Afghanistan, warning of the dangers of land mines.

[5] In the film, "The Story of the Little Carpet Boy," one puppet loses several limbs before he learns to avoid land mines completely.