Mendez' parents were Antonio Méndez Pomales, a native of Fajardo and Martina Rivera García from Naguabo. They moved from Puerto Rico to New York City seeking a better life. His father owned and attended a grocery store in the South Bronx while his mother cared for their eight children at home. When Mendez' mother became ill and the family's economic situation worsened, his father could not raise him and his siblings, therefore 2 were sent to foster homes and 6 were placed in the Mission of the Immaculate Virgin, an orphanage on Mount Loretto, Staten Island. There he received his primary and secondary education. Mendez was a member of the cadet corps along with his brothers and many of the "Mount" kids. At a young age, he became fascinated with military life and with his friends would often imagine that he was on a "patrol" while camping at Stokes State Forest and Worthington State Forest.