Before his accession to the throne at age 62, he served as crown prince for 42 years. During the long reign of his father, King Christian IX, he was largely excluded from influence and political power.

Frederick was born on 3 June 1843 in the Yellow Palace in Copenhagen as Prince Frederick of Schleswig-Holstein-Sonderburg-Glücksburg, a junior male line of the House of Oldenburg, which was descended from Christian III of Denmark and which had ruled as non-sovereign dukes in Schleswig-Holstein for eight generations, including Frederick's grandfather. Frederick's parents were Prince Christian of Schleswig-Holstein-Sonderburg-Glücksburg and Louise of Hesse-Kassel.

In 1853, his father was chosen as the heir presumptive to the Danish throne, because Frederick's mother, Louise of Hesse-Kassel, was a close relative of the last Danish king of the Oldenburg main line (the other heirs of the House of Hesse renounced their claims to the Danish throne in favour of Louise and her husband). Accordingly, Frederick became a Prince of Denmark in 1853.

After his confirmation in 1860, Frederick was given a military education. In 1863, Prince Frederick was sent to do studies at the University of Oxford but when his father ascended the throne in November that year, he became Crown Prince and returned to Denmark.

As Crown Prince of Denmark, he was given a seat in the State Council and subsequently assisted his father in the duties of government.

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Wikipedia