This randomized, double-blind, placebo-controlled, parallel-group study will evaluate the efficacy and safety of crenezumab versus placebo in participants with prodromal to mild AD. Participants will be randomized 1:1 to receive either intravenous (IV) infusion of crenezumab or placebo every 4 weeks (q4w) for 100 weeks. The primary efficacy assessment will be performed at 105 weeks. The participants who do not enter open-label extension will enter for a long term follow-up period for up to 52 weeks after the last crenezumab dose (Week 153).