Sickles was born in New York City on October 20, 1819. He was the son of George Garrett Sickles and Susan Marsh Sickles. At a young age he kept running away from home. His parents sent him to boarding school when he was 15 years old but he had to leave after a fight with a teacher. He became a printer's helper for a year before returning to New York City. There he developed the habit of hanging out with prostitutes and others with poor reputations. His parents decided he needed a good education and arranged for him to live in the Da Ponte household. Sickles was to be tutored by the elderly Professor Lorenzo Da Ponte. He was already friends with the younger Professor Da Ponte, who was in his 30s at the time. Sickles' parents hoped the friendship would be a good influence on Sickles. Maria Cooke, a young American girl also lived in the household. She was about the same age as Sickles. She was thought to be the natural daughter of the elder Da Ponte who adopted her. Her husband, Antonio Bagioli and their infant daughter, Maria, also lived with the Da Ponte family. At the time she was 3 years old, Sickles was 20. They would marry some 13 years later.