Marisol Valles was hailed as "Mexico's bravest woman" after she agreed to take the job in Praxedis Guadalupe Guerrero, a no-man's-land close to the Texas border, in October.
But she has since been targeted by a criminal gang that wanted to make her work for them.
After several months in the job she was forced to flee, along with two relatives, and will seek asylum in the US.
In December Erika Gandara, 28, the only police office left in the nearby town of Guadalupe was kidnapped and her house was set on fire. Her fate remains unknown.
The towns are in an area where the Sinaloa and Juarez drug cartels are fighting for control of smuggling routes into the US.
Miss Valles, a criminology student who has a baby son, was recruited after her predecessor was gunned down in July 2009.
The town had been unable to find anyone willing to take the job for more than a year.
She tried a novel approach to policing in Mexico, declining to carry a gun and sending female police officers door-to-door to build community trust.
After taking the job, she said: "Of course there is fear, I'm like any other human being, fear will always be there." Last year more than 3,000 people were killed in the nearby city of Ciudad Juarez.
The city is at the epicentre of Mexico's four year drug war which has left more than 35,000 dead across the country.
