Her father was a bricklayer and her mother, a teacher. She became a model after being spotted at the age of 16 during a national talent search.
She was featured on the cover of the 2003 Sports Illustrated Swimsuit Issue, and has also modeled for Victoria's Secret, and for many other companies in the women's clothing market. On June 3, 2004, she was one of the telecast judges during the 53rd annual Miss Universe competition in Quito, Ecuador.
During the Indian Ocean earthquake of 26 December 2004, Petra was at the Khao Lak resort in Thailand with her main photographer and fiancé, Simon Atlee when a tsunami struck where they were staying.
Atlee was killed by drowning, and Němcová suffered a broken pelvis and serious internal injuries, but she managed to hold onto the top of a palm tree, reportedly for eight hours, until she was rescued by Thai civilians and airlifted to an inland hospital nearby. Petra spent three weeks in a Thai hospital, and then flew home to the Czech Republic, where she spent an additional three weeks in the hospital. Atlee's body was not found until over two months after the tsunami. Petra's first interview after her near-death experience was with Diane Sawyer on ABC's Primetime Live, on March 9, 2005. She has also told of her experiences on the "Coady Read Live" program on CNN television.
Glamour magazine recently awarded Němcová a "Survivor Award" for the tsunami ordeal, and how she has picked up her life since then, her comeback being complete with her appearance in the 2006 Sports Illustrated swimsuit issue. She also appeared in the 2007 British FHM calendar as Miss May.
She wrote Love Always, Petra, ISBN 0446579130, published December 2005. It tells of her early life under Communist rule in Czechoslovakia, her entrance into the field of fashion and modeling, and her breakthrough into Sports Illustrated, which was her springboard to becoming a supermodel. She also tells of her life with Simon Atlee and how they fell in love and became engaged to marry.
On 7 July 2007 she presented at Live Earth.