We examine the efficacy of affirmative action at universities whose value depends on peer and alumni networks. We study an elite Brazilian university that adopted race- and income-based affirmative action at a large scale. Using employer-employee data, we show that a key benefit of attending the university is access to high-paying jobs affiliated with its alumni. Affirmative action increased disadvantaged students' access to these firms and raised their early-career earnings. But these benefits faded over time. Further, the increase in student body diversity lowered the job prospects and earnings of the school's most highly ranked students.