Astrup grew up in Ålhus in rural Jølster between the Sognefjord and the Nordfjord. He was the eldest son of eleven children born to Christian Astrup (1844–1919) who was a parish priest at Ålhus Church in Jølster. Astrup was the great-grandson of Nils Astrup (1778–1835), a member of the Norwegian Parliament.
Astrup's upbringing in Ålhus, a rural area near the Sognefjord and Nordfjord, was within the context of his family background. He was the eldest son of eleven children born to Christian Astrup (1844–1919), who served as a parish priest at Ålhus Church in Jølster. His lineage traced back to Nils Astrup (1778–1835), a member of the Norwegian Parliament.