Manuel Medina-Mora has long had ambitions of running Citigroup. He is getting closer.

After more than a decade as chief executive at Grupo Financiero Banamex, the Mexican banking company that is one Citigroup's choicest businesses, the 59-year-old Medina-Mora resigned last week to devote more attention to a daunting new job. The Mexican banker has to somehow turn around Citigroup's struggling consumer banking operations worldwide.