Assuming perspectives from the positivism philosophical school, his main work was Udsigt over den norske Historie, four volumes issued from 1873 to 1891.

[1] Sars was born in the parish of Kinn (now Flora Municipality) in Nordre Bergenhus Amt (now Sogn og Fjordane), Norway.

He was a cousin of architect Hjalmar Welhaven as well as a brother-in-law of explorer and scientist Fridtjof Nansen and musician Thorvald Lammers.

In 1856, he was awarded the Crown Prince's gold medal (Kronprinsens gullmedalje) for having written a prize-winning treatise on the Kalmar Union.

He wrote a pioneering work on Norway during the union with Denmark (Norge under Foreningen med Danmark), published in four parts between 1858 and 1865.

[2] After his publication of Historisk Indledning til Grundloven (Historical Introduction to the Constitution) in 1882, he was regarded among the Liberal Party's most central theoreticians.