Blehr grew up at a farm at Stange in Hedmark, Norway. His parents were Albert Blehr (1805–1872) and Maren Wilhelmine Ludovica Kathinka Stenersen (1818–1877).
His father was a doctor and physicist at Sanderud Hospital.
He graduated in 1865 and then began studying the University of Christiania. Blehr graduated cand.jur. in 1871. He served as parliamentary reporter for the newspapers Dagbladet and Bergens Tidende. In 1874, he was one of the founders of the Fjordabladet where he served as the first editor-in-chief until 1882. In 1878, he also started and served as the first editor of the Sogns Tidende. Blehr was the governor of Nordre Bergenhus (1883–1888) and for Nordland (1895–1900).[1][2]
In 1877, he established himself as a prosecutor at Lærdal in Sogn. In 1879 Blehr was elected as first deputy representative to the Storting for Nordre Bergenhus amt (now Sogn og Fjordane) and from 1883 to 1888 he was a permanent representative. In the fall of 1888 he was not re-elected to the Storting.
He became a prosecutor (fogd) in Sunnfjord and Nordfjord. In 1889 a lawyer in Hålogaland. He held this assignment until he became a judge (lagmann) at Kristiania in 1893. In 1894 Blehr was again elected to the Storting, now for Nordland. Blehr was re-elected as parliamentary deputy for Nordland in 1898. On 21 April 1902, he took over as Prime Minister of the Norwegian government in Kristiania.[3]
In October 1903, Blehr resigned as a result of an election defeat. In 1905 he was appointed as County Governor (stiftsamtmann) at Christiania (now Oslo), an office he held until 1921. On 21 June 1921, Otto Blehr became prime minister and at the same time also chief of the Ministry of Finance. He was also a member of the Norwegian delegation to the League of Nations 1920 and 1922–1925. On 3 March 1923 the government resigned.[4][5]