Cruz was born in San Juan, Puerto Rico where he was raised by his parents. His parents practiced brujeria and were followers of spiritism ("espiritismo"). They mentally abused him; his own mother would call him "Son of Satan". The neighborhood in which he lived was one of the worst in Puerto Rico and Cruz was always getting into trouble. According to the book "Run Baby Run", His parents sent him to live with his Brother in New York City as a teenager, and Cruz soon ran away and started living on the streets of the city. He became a member of the Mau-Mau street gang, and about six months later Cruz was elected Warlord of the gang. He quickly rose to be their leader.

Shortly after, a preacher by the name of David Wilkerson was preaching in the neighborhood when Cruz encountered him. The preacher told Cruz that Jesus loved him and would never stop loving him. A shocked Cruz responded by slapping Wilkerson and threatening to kill him. Wilkerson looked Cruz in the face and said that he could cut Wilkerson into a thousand pieces, but every piece would still say Jesus loves him. Wilkerson said that no one can kill love, and that God is love. That afternoon the preacher showed up at the Mau Mau's headquarters to repeat his message, and was slapped again by Cruz. Wilkerson just smiled, and then prayed for Nicky.

Two weeks later, Wilkerson had an evangelistic meeting in the neighborhood. When Cruz heard about it, he decided to go and teach the preacher a "lesson" and, with some of the members of his gang, he headed for the boxing arena where the rally was being held, on a bus sent specially by Wilkerson. According to Cruz, when he arrived at the arena, he felt guilty about the things that he had done and started to pray. Wilkerson preached, then asked the Mau Maus to take up a collection. Nicky sprang to his feet and led a group of the gang through the crowd insisting on people giving money. Going backstage, he saw an exit, but was struck by the fact that someone had actually trusted him. He gave the money to Wilkerson on stage. Later, Wilkerson gave an altar call, and many gang members responded. Wilkerson prayed with Cruz, and Cruz asked God to forgive him.

The following day Cruz and some of the gang members who converted went to the police and turned in all of their bricks, handguns and knives, shocking the police officers in the station. They said that if they had seen the group approaching, they probably would have shot them down. Cruz began to study the Bible and went to Bible College. He became a preacher and returned to his old neighborhood, where he preached and persuaded some of the Mau-Maus to accept Jesus, including the gang's new leader, Israel Narvaez.

Nicky Cruz has written two autobiographies, Run Baby Run, with Jamie Buckingham (1968), and Soul Obsession, with Frank Martin (2005). He has also written several books with a Christian theme, including The Corruptors (1974), The Magnificent Three (1976), and Destined to Win (1991). Cruz's conversion was depicted in the 1970 film The Cross and the Switchblade.