The 18-year-old royal made a solemn declaration to abide by the Constitution at the Council of State meeting.
The State Council is the body in the state government where all laws and important government measures are dealt with.
Wearing a navy suit and a patterned tie, Christian's big moment was witnessed by his father, Crown Prince Frederik and his grandmother, Queen Margrethe.
It also means that Prince Christian can act as regent if Queen Margrethe is ill or out of the country.
The declaration also marked the first time that three generations were present in the Council of State at the same time.
Queen Margrethe, 83, has reigned for over 51 years, while Crown Prince Frederik, 55, is her heir.
Crown Princess Mary was appointed to Denmark's Council of State in 2019, and was pictured alongside her husband and mother-in-law after signing a declaration.
Queen Margrethe's youngest son, Prince Joachim (who currently resides in Washington D.C. with his wife, Princess Marie), and the monarch's sister, Princess Benedikte, can also take on responsibilities.
Prince Christian celebrated his 18th birthday on 15 October, with Queen Margrethe throwing her grandson a lavish gala dinner at Christiansborg Palace.
The queen and Christian both gave speeches at the bash, and the night was rounded off with a performance by Danish rock star, Benjamin Hav and his band, The Family.
Watch the clip below for an insight into the celebrations…
Guests also included members of other royal households, including Princess Ingrid Alexandra of Norway, Princess Catharina-Amalia of The Netherlands, Princess Estelle of Sweden and Princess Elisabeth of Belgium.
On the day of his birthday, Queen Margrethe gifted Prince Christian the Order of Elephant – Denmark's highest honour.
Christian is Crown Prince Frederik and Crown Princess Mary's eldest child. They are also parents to Princess Isabella, 16, and 12-year-old twins, Prince Vincent and Princess Josephine.
Frederik met Australian-born Mary Donaldson during the 2000 Sydney Olympic Games, with the couple tying the knot in 2004.
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