He became a prominent landowner in Córdoba Department and in 1995 he joined the "Autodefensas Campesinas de Córdoba y Urabá" paramilitary group, citing being tired of guerrilla extortion and abuses that the Colombian authorities failed to prosecute.
The paramilitary groups in Colombia later expanded, and in April 1997 created what was known as "Autodefensas Unidas de Colombia" (AUC), an umbrella organization under the leadership of the "Autodefensas Campesinas de Córdoba y Urabá" led by Mancuso and Castaño.