Montero began his career in Uruguay with Peñarol in 1990, before moving to Italian side Atalanta in 1992. He joined Juventus in 1996, where he remained until 2005, winning four Serie A titles, among other trophies; he subsequently moved to Argentine club San Lorenzo. In 2006, he transferred back to Peñarol, where he retired in 2007. At international level, he represented the Uruguay national football team at the 1997 FIFA Confederations Cup, at the 2002 FIFA World Cup, and at the 2004 Copa América.