He won by capturing more than half the vote, according to early returns, more than any candidate since the nation began its transition to democracy nearly 20 years ago. In a reflection of the lopsided vote, his main competitors conceded the race within 45 minutes of the polls’ closing, another historical first.

With his coalition partners, it is likely that he will hold a majority in Congress, potentially giving him more power to enact his policies.

In his acceptance speech Sunday night in Mexico City, Mr. López Obrador sought to unite an electorate polarized over his election, and promised to look out for all citizens — with the poor being first among them.

“I call on all Mexicans to reconciliation, and to put above their personal interests, however legitimate, the greater interest, the general interest,” he said. “The state will cease to be a committee at the service of a minority and will represent all Mexicans, rich and poor, those who live in the country and in the city, migrants, believers and nonbelievers, to people of all philosophies and sexual preferences.”

A global repudiation of the establishment has brought populist leaders to power in the United States and Europe, and conservative ones to several countries in Latin America, including Colombia after an election last month.

“The recent elections in Latin America have exhibited the same demand for change,” said Laura Chinchilla, the former president of Costa Rica. “The results are not endorsements of ideologies, but rather demands for change, a fatigue felt by people waiting for answers that simply have not arrived.”

Mr. López Obrador, who vowed to cut his own salary and raise those of the lowest paid government workers, campaigned on a narrative of social change, including increased pensions for the elderly, educational grants for Mexico’s youth and additional support for farmers.