Hamer received a business education and was employed by a Dutch export company. In 1937 the company sent him to Venezuela; and in 1938 to Guatemala, where he joined World War II. In 1942 he returned to Germany and served in the army during the Russian campaign. In 1948, at the end of the war, he returned to Guatemala and established himself in El Salvador, where he created a company to import and distribute machinery and equipment.