Her father was a mason 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 but appeared also in the editions of 2001, 2003, 2004, 2005 and 2006. She has also modeled for Victoria's Secret, and for many other companies in the women's clothing market. In addition to working with world class photographers on her Sports Illustrated shoots, she was the object/subject of Joanne Gair's body painting work. On June 3, 2004, she was one of the telecast judges during the 53rd annual Miss Universe competition in Quito, Ecuador.

Glamour magazine awarded Němcová a "Survivor Award" in 2005 for the way she picked up her life following her tsunami ordeal; 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 is now the host of TLC's A Model Life.

She wrote Love Always, Petra (ISBN 0446579130) published December 2005, which 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.

Nemcova has also moved on romantically, dating British singer James Bluntin 2006. They have reportedly broken up since early July in 2007.

On July 7, 2007 she was a presenter at the American leg of Live Earth and appeared in the music video 'Misfit' by American indie group Elefant[2]. She is currently working with HP computers to produce a multi-million dollar TV and web campaign.

In July of 2007 Nemcova has become a vegan after learning about the planet’s rapidly depleting fish stocks. The Czech Republic born model gave up meat and dairy in February.

On November 12, 2007 Nemcova visited Haiti alongside Haitian-born musician Wyclef Jeanas part of a charity-sponsored event. The inauguration of a computer lab and an eatery in the slums of Port-au-Prince were funded and sponsored by Jean's Yéle Haïti foundation and Nemcova's Happy Hearts Fund.

Petra was sighted at a New Years Eve party on Kaua'i Dec 31 2007.