Rios grew up in New York City's South Bronx neighborhood and is of Puerto Rican descent. By all accounts from Pun's family, his early years were very difficult, including witnessing his mother's drug abuse, his father's death, and a stepfather who was very hard on Pun. According to his grandmother, Pun would become angry and self-destructive, punching holes in the walls of his family's apartment. He used video games as an outlet for his frustration. His favorite video game was Bad Dudes. Rios dropped out of high school and for some time was homeless staying in abandoned buildings or at friends' homes.
During the 1980s, Big Punisher was already a rapper. At this point Big Pun was operating under the alias Big Moon Dawg. Rios met fellow Puerto Rican and Rutland rapper Fat Joe in 1995 and made his commercial debut on Joe's second album, Jealous Ones Envy, in addition to appearing on a b-side to Joe's "Envy" single, "Fire Water" and "Watch Out."
Later, "I'm Not a Player" (featuring an O'Jays sample) was supported by a significant advertising campaign and became an underground hit.
In 1997 producer Knobody's production partner Sean C took advantage of his new role as A&R at Loud Records to play Knobody's tracks to Big Pun. Suitably impressed the rapper hired Knobody to remix "I'm Not a Player". The song, featuring Joe, became Big Pun's first major mainstream hit and major breakthrough for Knobody. The full-length debut Capital Punishment followed in 1998, and became the first album by a solo Latino rapper to go platinum, peaking at #5 on the Billboard 200. Capital Punishment was also nominated for a Grammy, but lost out on the award to Jay-Z's Vol. 2: Hard Knock Life.
Big Pun became a member of Terror Squad, a New York-based group of rappers founded by Fat Joe, with most of the roster supplied by the now-defunct Full a Clips Crew who released their debut album The Album in 1999.