Dahl was born on 14 March 1778 at Nedre Fet, Uskedalen, in Kvinnherad. He was the second oldest of the 12 children of lieutenant colonel Johan Koren Dahl (1743 - 1831) and Else Alstrup (1757 - 1838). Niels was educated by the vicar Niels Hertzberg, graduated from Bergen Katedralskole in 1796, and studied theology in Copenhagen from 1797 to 1801. He then worked as an assistant to Hertzberg in his home village. There he met a relative of the vicar, Hylleborg Hertzberg (1765 - 1848), that he married in 1805.
Dean and farmer
From 24 February 1894 Dahl worked as a curate in the Eivindvik parish, and as vicar from 18 December 1807. In 1823, he became dean of Ytre Sogn. Dahl did much to improve agriculture in the parish. He started with the vicarage, and by means of new agricultural methods and building traditions, he gradually turned the vicarage into a model farm. Over a period of 20 years, he was also behind land reallocation for close to 200 holdings in the parish. In this connection, he helped many farmers to buy their houses.
For his own private means, Dahl educated teachers, as well as establishing and running the first two schools in the parish. By changing the school tax system, he saw to it that teachers were paid enough to make a living. In 1829, he established the Gulen public library, and 134 of these books came from his private collection. He took personally care of the education of the confirmation candidates. On one occasion, all the candidates and 10 teachers stayed in his home and Dahl gave them education, room and board.
With an improved accounting of the poor relief system, he turned the poor relief into a model organization. The income came from the fines by the bailiffs and the tax collectors. Dahl looked upon liquor as the biggest obstacle to fighting poverty. Innkeepers and other licences persons were fined when they sold illegally, or taxed so hard that some of them went bankrupt. In addition, Dahl opened his home for sick people, and he vaccinated almost the whole parish twice against smallpox. He also saw to it that the first midwife in Ytre Sogn was employed about 1827.
Mayor and representative to the Storting
Dahl became mayor when the Board of Aldermen Acts were passed in 1837, and held this office until 1849. He was a representative to the Storting (Parliament) in 1814, 1824, and 1827, and he was held to be the strongest anti-Swede of all the representatives, and he was an ardent speaker in all the debates on the Constitution.
Memorial at Eivindvik
The dean's residence was called "Herresalen". The first floor has been used as a meeting room for the local council from 1837 until fairly recently. The vicar's quay with stairs leading up to the church and the 1.5.metre-high wall encircling the infields of the vicarage still stand. Every year, the youth association of Eivindvik organizes the "Dahl days", where various aspects of Dahl's life and work are covered. A memorial for Dahl by the artist Elisabeth Steen was erected on 4 August 2002, 150 years after his death.