She is known for her work on the television series Mad Men and her film adaptation of American Woman based on Susan Choi's novel of the same name.

Chellas adapted Linda Spalding’s Who Named the Knife as a television movie entitled Murder on Her Mind; she also executive produced.

She also wrote The Life Before This, a low-budget feature directed by Jerry Ciccoritti that premiered at TIFF in 1999; she also served as an associate producer.

Her script for the Showtime/CBC television movie Restless Spirits (Canadian title: Dead Aviators) garnered her both a Gemini and a day-time Emmy nomination for Outstanding Writing in a Children's Special.

Together with her colleagues on the writing staff, Chellas was nominated for the Emmy Award for Outstanding Drama Series for Mad Men in 2013, 2014, and 2015.

Semi Chellas wrote the screenplay for the movie Ophelia, directed by Claire McCarthy, which premiered at Sundance in January 2018.

It won the Special Jury Prize for lead actresses Chau and Gadon at the Calgary International Film Festival in 2019.

Chellas has mentored and taught at Maisha Film Lab of East Africa, Toronto Workshop for Underprivileged Youth, Tribeca Festival's Through Her Lens, and at the Sundance Institute.

Chellas received the Inaugural award for Creative Excellence in 2014 at the Canadian Film Centre (presented by Norman Jewison).

Her interest in social change fuels much of her writing, including her Emmy nominated work on Mad Men, and her directorial debut American Woman.