Daniel Bernoulli. I am not sure if he counts, because he knew he wanted to study mathematics. Yet his formal education was in business and medicine. According to Wikipedia,

"Around schooling age, his father, Johann Bernoulli, encouraged him to study business, there being poor rewards awaiting a mathematician. However, Daniel refused, because he wanted to study mathematics. He later gave in to his father's wish and studied business. His father then asked him to study in medicine, and Daniel agreed under the condition that his father would teach him mathematics privately, which they continued for some time."