MEXICO CITY—Andrés Manuel López Obrador, a leading contender for Mexico’s presidency in 2018 who casts himself as an honest figure fighting a corrupt political mafia, failed to disclose two apartments he owns in his declaration of assets released in August.

The leftist recently joined growing numbers of Mexican politicians in releasing a full accounting of their wealth, following public pressure from a series of conflict-of-interest scandals that have dogged the administration of President Enrique Peña Nieto.

Mr....