Unlike many other painters, he is not educated from the Danish Art Academy. He is a self-taught painter and artist. In 1959 he attended a teacher seminar in Hellerup. Prior to this he had taken an ordinary college degree. However, he already had an interest in visual art, and he debuted his very first art exhibition in 1963. He had a new and simple style, aligned with European sensibilities and particularly Danish modernism, leading to many exhibitions here in the country and abroad. He quickly became popular. His art consists of large areas that are broken by a few attributes. The vibrant works are made in bold colors. Arrnoldi's works are very graphic. This has led him throughout his career to design posters for companies in Denmark and abroad. He has designed posters for well-known clients such as Volvo Denmark, DSB, British Airways, Lincoln Center New York and Politiken.