Reintroducing native wildlife populations is a common conservation-management approach aimed at reducing the threat of extinction and restoring ecosystem function. Captive-born individuals are sometimes used in reintroductions, but the effectiveness of this strategy is poorly understood due to insufficient post-reintroduction monitoring and evaluation. Our objective was to evaluate the utility of using captive-born individuals of an endangered rodent, the Perdido Key beach mouse (Peromyscus polionotus trissyllepsis) to reestablish a population on the western end of Perdido Key at Gulf State Park, Alabama, from which it was extirpated in 1997. We released 48 captive-born mice in March 2010 and monitored the population through livetrapping across 8 sessions spanning 5 years. We evaluated temporal changes in microsatellite genetic diversity to determine whether mice born in the wild were derived from released mice. The number of mice declined by 73% to 13 individuals in the first 2 weeks after release but increased to an estimated 206 (95% confidence interval = 195–217) individuals after 5 years. Genetic monitoring demonstrated a slight decrease in diversity during the first 3 months but an increase by year 5. Admixture from a neighboring population was detected in year 2 and year 5, which corresponded to the largest increase in population size between trapping sessions. This change in abundance corresponded with a doubling of Ne/N signifying a possible role of admixture in population growth and resilience. This study demonstrates the feasibility of using captive-born beach mice to reestablish populations when wild populations are too small to serve as donors for a translocation. We could not, however, discern how gene flow from an expanding neighboring population affected growth or persistence of the reintroduced population. Our study emphasizes that in future reintroductions, genetics should be monitored together with demographic patterns, because cryptic gene flow could affect how we interpret reintroduction success.