Hornsrud was born in 1859 to Gunhild Dorthea and Anders Christophersen at the Horsrud farm in Skotselv, Eastern Norway, which had belonged his father's family in generations.

While his home only had religious literature, a local library in Hokksund provided him with a wider set of books and also the weekly magazine Skilling-Magazin.

[4] After confirmation, he stayed to work at the family farm while his elder brother studied at Jønsberg Agricultural School.

[7] Working in the shop brought him in contact with a wide array of local townspeople and farmers from neighboring areas and with the political discussion of the time.

The program of the latter included universal suffrage, no tariff for basic goods, progressive taxation and better primary education.

[15] He had become partly disillusioned with political work due to internal strifes and accusations that he was a "minister socialist" with too much sympathy for the Liberal party and seeking too much power in this own hands.

[16] When Torgeir Vraa was elected to the Storting in 1905, Horsrud became interim editor of the Labour Party newspaper Fremtiden in Drammen.

[19] The 1927 parliamentary election was a victory for the Labour Party which won 59 of the 150 seats and became the parliament's largest group.

He recommended that the King ask the leader of the Centre Party, Johan Mellbye, to form a new cabinet.

When Mellbye's attempt failed, the King called the Storting's president Carl Joachim Hambro and vice-president Hornsrud for consultation on 23 January.

[20] On the advice of Hornsrud, the King contacted the leader of the Labour Party parliamentary group, Alfred Madsen.

When Madsen subsequently asked the group whether the party should accept to form a cabinet, Hornsrud was among those who advocated strongest for a positive response.

Governor of the Central Bank of Norway Nicolai Rygg requested that the government should pay for a guarantee fund for the banks and when Hornsrud declined, Rygg petitioned the leader of the Liberal Party Johan Ludwig Mowinckel and other non-socialist party leaders to cause the downfall of the Hornsrud cabinet.

[24] Hornsrud became a parliament appointed member of the chair (direksjonen) for Norges Hypotekbank in 1926, a bank which was designated to provide cheap loans to the agricultural sector.