On January 29, 1973, Frontier Airlines hired her. It was fifteen years after her first plane flight. This marked an opening for American women in one of the last sex-segregated occupations in the civilian aviation industry. When Warner was hired there were no other women working as pilots for the major commercial airlines. By 1978, there were about 300 female commercial pilots in the United States.
On February 6, 1973, Howell Warner served for the first time as second officer on a Frontier Airlines Boeing 737. The flight departed from Denver's Stapleton Airport for Las Vegas. Within six months, she was promoted to first officer. In 1974, she became the first woman member of the Air Line Pilots Association (ALPA).