<p>The relationship between job security and politicians’ legislative effort is bidirectional, making isolating the impacts, and the mechanisms underlying them, in either direction difficult. Increased legislative effort could increase politicians’ job security, as they might be considered to be more desirable by voters, however, increased job security can change the incentives to engage in legislative activity. We analyse the behaviour of UK Members of Parliament (MPs) from 1997 to 2019 to examine the mechanisms driving any impact of job security on legislative effort, using an instrument for job security that is not impacted by individual effort. We show that the impact of job security on effort is primarily driven by an opportunity cost of time allocation mechanism, with MPs who are less secure in their job allocating less of their time to legislative activities. This opportunity cost increases with the breadth of their legislative effort, and is primarily driven by individuals with lower outside options, who face a greater cost of electoral defeat. We provide reduced-form evidence that this opportunity cost is primarily driven by the maintenance of non-parliamentary human capital by MPs, rather than activities designed to improve their electoral prospects.</p>