He was born the second-eldest son of Christian IV and Anne Catherine of Brandenburg. The traditional loyalty of the Danish middle classes was transformed into a boundless enthusiasm for the king personally, and for a brief period Frederick found himself the most popular man in his kingdom. Frederick was born at Haderslev, in Schleswig, son of Christian IV and Anne Catherine of Brandenburg. As a young man, he demonstrated an interest in theology, natural sciences, and Scandinavian history. Frederick was educated at SorÃ¸ Academy and studied in the Netherlands and France. On October 1, 1643 Frederick wed Sophia Amelia of Brunswick-Lüneburg, whose energetic, passionate and ambitious character was profoundly to affect not only Frederick's destiny but the destiny of Denmark.
He was born the second-eldest son of Christian IV and Anne Catherine of Brandenburg. King Christian V of Denmark and Norway was born the eldest son of King Frederik III of Denmark and Norway and Sophie Amalie of Brunswick-Lüneburg, the daughter of Georg, Duke of Brunswick-Lüneburg and Anne Eleonore of Hesse-Darmstadt. In the beginning of May, the still pending negotiations with that power were broken off, and on 1 June Frederick signed the manifesto justifying a war, which was never formally declared. He eventually succeeded in removing the two most influential members of Rigsraadet from office in 1651: his brothers-in-law Corfitz Ulfeldt and Hannibal Sehested.  At the age of eighteen, he was the chief commandant of the Bremian fortress of Stade. Frederick was only considered an heir to the throne after the death of his older brother Prince Christian in 1647. Frederik wanted the title of "Prince of Westphalia" but the emperor, Gustavus Adolphus, was reluctant to agree and preferred "Governor." Frederick III at once sued for peace; and, yielding to the persuasions of the English and French ministers, Charles finally agreed to be content with mutilating instead of annihilating the Danish monarchy; the Treaty of Taastrup, on February 18 and the Treaty of Roskilde, on February 26, 1658).  He was a reserved and enigmatic prince who seldom laughed, spoke little, and wrote less, a striking contrast to Christian IV. The Dutch then assisted in the liberation of the Danish Isles in 1659. Daily entries from the 17th century London diary. Sein jüngerer Bruder war Georg I. von Griechenland. The first Danish theatre, Lille Grönnegade, was created and the great dramatist Ludvig Holberg began his career.  He was then appointed commander in the royal shares in the Duchies of Schleswig and Holstein by his father. On 23 April he received the assent of the majority of Rigsraadet to attack Sweden's German dominions. The Swedish king traversed all the plans of his enemies by his passage of the frozen Little and Great Belts, in January and February 1658 (see Charles X of Sweden). In 1665, the Kongeloven (Lex Regia) was introduced: the “constitution” of Danish absolute monarchy, and the first assertion of divine right underpinned by a written constitution in Europe. But with all his good qualities Frederick was not the man to take a clear view of the political horizon, or even to recognize his own and his country's limitations. In his youth Frederick served successively as bishop coadjutor (i.e., assistant bishop with the right of succession) of the German dioceses of Bremen, Verden, and Halberstadt. He commanded Danish forces in Schleswig-Holstein during Denmark’s disastrous war with Sweden (1643–45) and succeeded to the throne shortly after the death (1648) of his father, Christian IV, agreeing to a charter that reduced the royal prerogatives.
Later that year, Frederick used his popularity to disband the elective monarchy in favour of absolute monarchy, which lasted until 1848 in Denmark. This was Frederick's first collision with the Danish nobility, who afterwards regarded him with extreme distrust. By the beginning of September, all the breaches were repaired, the walls bristled with cannons, and 7,000 men were under arms. Terror was the first feeling produced at Copenhagen by the landing of the main Swedish army at Korsør on Zealand. During the Torstenson War of 1643–45, Frederick lost control of his possessions within the empire. The peace banquet (Fredstaffelet) at Frederiksborg Castle following the signing of the Treaty of Roskilde in 1658. He also paid a visit to the dowager grand-princess Violante at the grand-ducal court of the Medicis, where the irreverent king was taken with the young dowager going as far as to refuse to leave the room while she was changing clothes. That form of faith would rise to prevalence during the reign of his son. The conclusion of peace was followed by a remarkable episode. New men came into government, which was marked by a rivalry between the ministers and councillors like Hannibal Sehested and Kristoffer Gabel. But his visit there was cut short by a message telling of his brother Christian's serious illness (he had, in fact, already died in Ulm). , In his youth, Frederick became the instrument of his father's political schemes in the Holy Roman Empire.
He also governed under the name Frederick II as diocesan administrator (colloquially referred to as prince-bishop) of the Prince-Bishopric of Verden (1623–29 and again 1634–44), and the Prince-Archbishopric of Bremen (1635–45).  Also offended by the countess's elevation were King Frederick's younger unmarried siblings, Princess Sophia Hedwig (1677–1735) and Prince Charles (1680–1729), who withdrew from Copenhagen to their own rival court at the handsomely re-modelled Vemmetofte Cloister (later a haven for dowerless damsels of the nobility).. Terror was the first feeling produced at Copenhagen by the landing of the main Swedish army at Korsør on Zealand on 17 July 1658. , So strong was the city by this time that Charles X, abandoning his original intention of carrying the place by assault, began a regular siege.