Born in Helsinki, Berglund studied the violin as a child, and played an instrument made by his grandfather. By age 15, he had decided on music as his career, and by 18 was playing in restaurants. During the Second World War, Berglund worked at the iron factories in Billnäs. Children were moved out of Helsinki during heavy stages of the war. His professional career as a violinist began in 1946, playing the whole summer at the officers' mess (Upseerikasino) in Helsinki. He had already played in dance orchestras in 1945. Formal study took place in Helsinki at the Sibelius Academy, in Vienna and in Salzburg. He was a violinist in the Finnish Radio Symphony Orchestra from 1949 to 1958 in the 1st violin section, unique among the instrumentalists in being accommodated for seating to account for the fact that he played the violin 'left-handed'.