Ernst was born in Winterthur, Switzerland on 14 August 1933 to Robert Ernst and Irma Ernst-Brunner. He was the oldest of three children of Irma Brunner and Robert Ernst. He grew up in a house built in 1898 by his grandfather, who was a merchant. During his childhood, he was interested in music, playing the violoncello and even considering a career as a musical composer. At 13-years old, Ernst stumbled upon a box of chemicals belonging to his late uncle, a metallurgical engineer. Young Ernst was excited by what he found, and set about trying all conceivable reactions, some of which resulted in explosions that terrified his parents.