Understanding the factors that allow a species to expand its range and adapt to changing habitats is essential for mitigating anthropogenic change. We evaluated how behavior and cognition facilitate colonization of new environments and evolve post establishment during natural biological invasions. Marine threespined sticklebacks are expert colonists with a penchant for invading freshwater environments and rapidly adapting to them. However, the role of behavior in facilitating rapid adaptation in this system has received little attention. By rearing replicate populations of sticklebacks under common garden conditions in the lab, we tested the hypothesis that boldness is favored in dispersers and that neophilia and flexibility are favored in recently-arrived immigrants. We found that dispersing populations comprised bold individuals, while sticklebacks from the invaded region were flexible in their behavior. Moreover, boldness and flexibility were negatively correlated with each another at the individual, family and population levels. Multiple lines of evidence suggest that there is a heritable component to boldness and flexibility, therefore their divergence is likely to be evolutionary in origin. If boldness is favored in invaders during the initial dispersal stage, while flexibility is favored in recent immigrants during the establishment stage, then the link between boldness and flexibility could generate positive correlations between successes during both the dispersal and establishment stages, and therefore play a key role in facilitating colonization success in this important model organism.