He graduated with the candidate.jur.
He was Prime Minister of Norway from 14 October 1895 to 17 February 1898 and from 22 October 1903 to 11 March 1905. As a legal scholar, he is known for his contributions to the development of public international law, and was chairman of the Institut de Droit International. degree at the Royal Frederick University in 1876, received a grant to study abroad, and became a research fellow at the Royal Frederick University in 1879. He obtained the dr.juris degree in 1885, and was professor of law at the Royal Frederick University from 1887 to 1906.
He was minister of justice in the Second cabinet Stang from 2 May 1893 to 14 October 1895.
In August 1895 he was Finance minister. Following his two bouts as Prime Minister, he entered the diplomacy in 1906, as ambassador to Copenhagen, The Hague, and Brussels.
From 1916 he was ambassador in Stockholm. Hagerup was passionately involved in the development of public international law.
In 1907 he headed the Norwegian delegation at the second peace conference in The Hague.
He was also delegate at international conferences regarding admiralty law. In 1920 he led the Norwegian delegation when the League of Nations convened for the first time in Geneva. The same year he was elected to the Law committee under the League council.
In 1888, he founded Tidsskrift for Retsvidenskab (Journal of Jurisprudence), and served as its editor until his death.
Royal Swedish Academy of Sciences]
He was a member of the Storting from 1901 to 1906. From 1897 he was member of the Institut de Droit International, of which he became the chairman in 1912. Hagerup was also member of the Norwegian Nobel Committee from 1 January 1907 until his death.