Federal civil servants need policy expertise to formulate and implement public policy. Presidents and congresses do not confer such expertise when they delegate responsibility for policy making. Existing research suggests that elected officials’ efforts to gain control of federal agencies can reduce agency expertise by inducing civil servants to exit the agency and reducing incentives for civil servants to exert effort, including acquiring policy expertise. I use data from an original survey of federal executives to examine whether politicization reduces agency expertise. I find that civil servants whose policy preferences diverge from those of political appointees are more likely to perceive that their agency is politicized and that civil servants who perceive their agency is politicized are more likely to exit the agency and less likely to engage in behaviors that build policy expertise. In total, these findings provide systematic, microlevel evidence demonstrating how politicization can reduce agency policy expertise.