Despite international commitments to end female genital mutilation/cutting (FGM/C), very little is known about the effectiveness of national policies in contributing to the abandonment of this harmful practice. To help address this gap in knowledge, we apply a quasi-experimental research design to study two west African countries, Mali and Mauritania. These countries have marked similarities with respect to practices of FGM/C, but differing legal contexts. A law banning FGM/C was introduced in Mauritania in 2005; in Mali, there is no legal ban on FGM/C. We use nationally representative survey data to reconstruct trends in FGM/C prevalence in both countries, from 1997 to 2011, and then use a difference-in-difference method to evaluate the impact of the 2005 law in Mauritania. FGM/C prevalence in Mauritania began to decline slowly for girls born in the early 2000s, with the decline accelerating for girls born after 2005. However, a similar trend is observable in Mali, where no equivalent law has been passed. Additional statistical analysis confirms that the 2005 law did not have a significant impact on reducing FGM/C prevalence in Mauritania. These findings suggest that legal change alone is insufficient for behavioral change with regard to FGM/C. This study demonstrates how it is possible to evaluate national policies using readily available survey data in resource-poor settings.