His studies at the Conservatory (1930-1938) included violin, viola, and counterpoint. He was a frequent ensemble performer. He debuted with Enescu's Concert piece for viola and piano (1935), and played viola for the Swedish premiere of Schoenberg's Pierrot Lunaire (1937). He applied twice for the Jenny Lind scholarship, which he was awarded in 1938 for study abroad in 1939. His first compositions date from this period and are exlusively small chamber works, most notably, the six Songs of 1935. In February 1939, he auditioned successfully for a viola position in the Stockholm Concert Society Orchestra (later Stockholm Philharmonic), but was immediately granted leave to use his scholarship to study viola with Maurice Vieux in Paris.
Between 1940 and 1950 Pettersson played viola in the Stockholm Concert Society Orchestra and in various Swedish Radio ensembles. Their repertoire at this time included a noteworthy amount of 20th-century music and the conducting services of many celebrated foreign conductors. Pettersson earned a reputation as a fine violist but a difficult person. He continued his studies, with Otto Ohlsson (counterpoint), Karl-Birger Blomdahl (composition), and Tor Mann (orchestration). His compositions from this time include the Barfotasånger and his first large-scale works: the Violin Concerto no. 1 (violin and string quartet), and Concerto for string orchestra no. 1. The first public performance of his music came in September 1950: Fugue in E, for oboe, clarinet and bassoon. In 1943 he married Gudrun Gustafsson. They moved in 1946 into a small apartment in southern Stockholm that was their permanent residence for the next 30 years.
By 1950 Pettersson had realized that he wanted to devote himself to composition. Granted leave from his orchestral position, he traveled to Paris for composition class studies with Honegger, and Milhaud, and private studies with René Leibowitz. His first commission came from the Swedish Radio Service in 1952 and was fulfilled by the Symphony no. 2. His string-orchestra concerto was performed at the Cologne Festival in 1953, but was overshadowed by the compositions of more "radical" composers such as Stockhausen. Performances of other compositions were infrequent and failed to generate much enthusiasm from the critics. He resigned his orchestral position in November 1952. Also at this time he began to experience the pain in his joints that later would be diagnosed as chronic rheumatoid arthritis.
Over the next 15 years Pettersson very gradually began to achieve recognition as a composer. There were new commissions from Swedish Radio, state composition awards, and in 1963, his first recording (Mesto from the string-orchestra concerto no. 3). In 1964 he was awarded a state income guarantee. The major breakthrough, however, occurred with the premiere of his seventh symphony by the Stockholm Philharmonic 13 October 1968. The performance received a standing ovation, the composer four curtain calls. He also was named an honorary member of the orchestra. His arthritis, however, had continued to worsen. This important concert was the last he was able to attend.
Deteriorating health forced Pettersson to be hospitalized for nearly nine months beginning in September 1970. Despite this nearly-fatal illness, the composer continued to work. He recovered sufficiently to be released, but thereafter was confined to his apartment. The international release of the recording of his 7th symphony finally extended his reputation beyond Sweden. A performance of the work was scheduled for an international tour by the Stockholm Philharmonic, but was cancelled at the last minute. The angry composer banned the orchestra from performing his music (June 1975, lifted one year later). More awards for composition were followed in 1976 by a new government-sponsored residence. There were also a series of television documentaries and three ballets by Birgit Cullberg to existing music. The American premiere of Pettersson's music came in October 1977 with a performance by the Baltimore Symphony and Sergiu Comissiona of the 8th symphony. While working on his 17th symphony the composer's declining health forced him again into the Karolinska Hospital (Stockholm) in May 1980. He died there 20 June 1980.