He made several voyages to the White Sea and to places in northern Russia, and in 1621 entered the service of the thriving Danish Icelandic Company.
It was he who obtained privileges for the burgesses of Copenhagen which placed them on a footing of equality with the nobility; and he was the life and soul of the garrison till the arrival of the Dutch fleet practically saved the city.
These eighteen months of crisis established his influence in the capital once for all and at the same time knitted him closely to Frederick III, who recognized in Nansen a man after his own heart, and made the great burgomaster his chief instrument in carrying through the anti-aristocratic Revolution of 1660.
His greatest feat was the impassioned speech by which, on 8 October, he induced the burgesses to accede to the proposal of the magistracy of Copenhagen to offer Frederick III the realm of Denmark-Norway as a purely hereditary state.
Whether he subsequently regarded the victory of the monarchy and its corollary, the admittance of the middle classes to all offices and dignities, as a satisfactory equivalent for his original demands; or whether he was so overcome by royal favour as to sacrifice cheerfully the political liberties of his country, is a matter for conjecture.