Horacek graduated with a BA from the University of Melbourne in 1991, majoring in Fine Arts and English. She then studied for a Diploma in Museum Studies.

She lives in Canberra, ACT.

Horacek started her career as a writer, and was a member of a community writing group in North Melbourne. Words are an important part of her cartoons, and sometimes dominate the pictures.

After illustrating a children's picture book for Mem Fox, she decided to write and illustrate her own children's books.

"My life has been a quest to find new and better place to stick cartoons", Horacek has said. Accordingly, her cartoons can be found in newspapers and magazines, online, on aprons and teatowels, on mugs, on fridge magnets and cards, and as limited edition prints. Her cartoons have been described as whimsical and quirky. As she says, "I take every day situations and make them strange"

It was her interest in feminism which "drove Horacek's early work and established her reputation as a cartoonist". Since then, in addition to an ongoing interest in women's issues, her cartoons have covered a wide range of social and political issues such as the Australian Republican Movement, immigration, indigenous issues and FlyBuys. Cartoonist Peter Nicholson describes her work as follows:

Horacek's first commissioned work for The Age newspaper was published on International Women's Day 1995, next to the obituary of Senator Olive Zakharov. This was her cartoon, Woman with Altitude, a work which has since appeared on fridge magnets greeting cards, tea-towels and T-shirts. In 2007, she said that "The woman with altitude ... represents who we could be". At various times she has had regular spots in such newspapers and magazines as The Age, The Weekend Australian Magazine, The Canberra Times and the Australian Book Review.

She illustrated Mem Fox's non-fiction book, Reading Magic, and in 2004 she illustrated her first children's book, Mem Fox's Where is the Green Sheep. It was shortlisted for several book awards, and in 2005 won the Children's Book Council of Australia Book of the Year - Early Childhood Award. She has since started writing her own children's books, the first being The Story of Growl (2007).