From 1819 to 1821 Fearnley studied at the Royal art college in Oslo and until 1823 at the academy of arts in Copenhagen, probably under Chr.A. Lorentzen (1749-1828). From 1823 to 1827 he studied at the academy of arts in Stockholm under C.J. Fahlcrantz (1774-1861).
Already during his time as a student he made many journeys, e.g. 1824 and 1826 through Norway and 1825 through Central Sweden. In 1829 he moved to Dresden where he was a pupil of J.Chr.C. Dahl (1788-1857) and where he met C.D. Friedrich (1774-1840).
From 1830 to 1832 he stayed in Munich. There he caused quite a sensation with his works of art, and there he met Chr. Morgenstern (1805-1867).
Autumn 1832 he travelled to Rome, where he stayed in contact with the artists around Thorvaldsen. Until 1835 Fearnley made study trips through Italy and then he travelled via Switzerland to Paris and London, being very impressed by J. Constable (1776-1837) and W. Turner (1775-1851).
After a short stay in Norway Fearnley lived and worked in England again from 1836 until 1838. There he made landscape paintings especially of the Northern English lake area.
Then he stayed again for two years in Norway. September 1841 he moved to Munich.
Fearnley belongs - as well as Dahl - to the most important painters of Norway and he is regarded as a master of the "paysage intime". His style of landscape painting - close to nature, with preference for many different forms of landscape and vegetation - had lasting influence on many artists in Munich and many of them stayed open to a Nordic painting style until the end of the 30s in the 19th century.