Riding a wave of populist anger fueled by rampant corruption and violence, the leftist Andrés Manuel López Obrador was elected president of Mexico on Sunday, in a landslide victory that upended the nation’s political establishment and handed him a sweeping mandate to reshape the country.

Mr. López Obrador’s win puts a leftist leader at the helm of Latin America’s second-largest economy for the first time in decades, a prospect that has filled millions of Mexicans with hope — and the nation’s elites with trepidation.