Past research on growth mindsets has focused on the benefits of viewing the self as flexible rather than fixed. We propose that employees can make more substantial agentic changes to their work experiences if they also hold growth mindsets about their job designs. We introduce the concept of dual-growth mindset—viewing both the self and job as malleable—and examine its impact on employee happiness over time. We hypothesize that fostering a dual-growth mindset yields relatively durable gains in happiness, while fostering a growth mindset about either the self or job is insufficient for sustainable increases in happiness. We tested these predictions using two experimental studies: a field quasi-experiment in a Fortune 500 technology company and a controlled experiment with employees in a variety of organizations and occupations. Across the two experiments, fostering dual-growth mindset yielded gains in self-reported and observer-rated happiness that lasted at least six months. Fostering growth mindsets about either the self or job alone did not generate lasting increases in happiness. Supplementary mediation analyses suggest dual-growth mindsets boosted happiness by enabling employees to plan more substantial job crafting. Our research suggests that durable gains in happiness at work depend on holding flexible mindsets about the job, not only the self.