Cornelius was born in Philadelphia to Sarah Cornelius () and Christian Cornelius. His father immigrated from Amsterdam in 1783 and worked as a silversmith before opening a lamp-manufacturing company. He attended private school as a youth and took a particular interest in chemistry. In 1831, he began working for his father and specialized in silver plating and metal polishing. Soon after the daguerreotype was invented, Cornelius was approached by Joseph Saxton to create a silver plate for his daguerreotype of Central High School in Philadelphia. Saxton's photo is believed to be the oldest photograph taken in the United States. It was this meeting that sparked Cornelius's interest in photography.