He later studied under Joachim Frich and Johannes Flintoe at the Royal School of Drawing from 1850, in Düsseldorf from 1854 to 1859, and in Paris under Thomas Couture (1859-1860) and Gustave Courbet (1861-1862).
After a visit to Italy he returned home in 1864 with a view to becoming an artist with a strong focus on archeology and history.
[2] Isaachsen was a knowledgeable and courteous man, a mixture of observation and locals, who spoke fluent German, Dutch, English, French and Italian.
Two years later his four-year-old daughter Johanna Marie died of diphtheria, and he was left with three small boys.
He was the paternal grandfather of Sigurd Daniel Isaachsen Willoch, an art historian and director of the National Gallery of Norway.