Niels Juel was born in Christiania (now Oslo), Norway, where his family sought refuge during the 1627 invasion of Jutland during the Thirty Years' War, while his father took part in the defense of the country at home.

From 1654 to 1656, he attended Admiral Michiel de Ruyter on two trips to the Mediterranean during engagements against North African pirates.

He served with distinction during the Dano-Swedish War (1658–60) and took a prominent part in the defence of Copenhagen against the forces of King Charles X of Sweden.

[8] During fifteen years of peace, Juel, as admiral of the fleet, labored assiduously to develop and improve the Royal Dano-Norwegian Navy, though he bitterly resented the setting over his head in 1663 of Vice-Admiral Cort Adeler (1622–1675), on his return from service to the Republic of Venice during the Turkish wars.

[9] Juel then won a European reputation, and raised Danish sea-power to unprecedented eminence, by the system of naval tactics, which consisted of cutting off a part of the enemy's force and concentrating the whole attack on it.

He first employed this maneuver at the Battle of Jasmund off Rügen (25 May 1676) when he broke through the enemy's line in close column and cut off five of their ships, despite nightfall prevented him from pursuing them.

Juel took a leading part in Cornelis Tromp's great victory off Battle of Öland (1 June 1676), which enabled the Danes to invade Scania unopposed.

For this great triumph, the reward of superior seamanship and strategy at an early stage of the engagement, Juel's experienced eye told him that the wind in the course of the day would shift from S.W.