Hasslev was born on 1 November 1926 in Laholm, Sweden, the son of Filip Pålsson, an odontology doctor, and his wife Marta (née Ljungberg).
[1] Hasslev conducted his clerkship at Vättle, Ale and Kullings Judicial District from 1953 to 1955 and was assistant at the Ministry of Commerce and Industry in 1955.
[2] Hasslev was also secretary in several government inquiries from 1955 to 1960, in the Delegation for the Atomic Energy Questions 1961-1964, served in the Committee of Supply from 1959 to 1962 and was an expert and representative of trade and custom negotiations.
[1] Hasslev was also vice chairman of the traffic policy investigation, chairman of the transport research delegation, the traffic planning investigation, the 1973 negotiating team regarding the Öresund link and the 1975 Öresund delegation.
He was a member of the Nordic Road Safety Council (Nordiska trafiksäkerhetsrådet), the board of the AB Aerotransport and the Swedish Ships' Mortgage Bank (Svenska skeppshypotekskassan).