In December 1883 Sepúlveda left Los Angeles and moved to Mexico City, where he spent thirty years practicing law and became the administrator of jurisprudence and legislation under the administration of President Porfirio Diaz of Mexico. In 1895 he was made the first secretary and chargé d'affaires of the United States in Mexico. While in that country, in 1905 he helped organize the Guadalupe Council of the Knights of Columbus.
