Kaarle was born on July 2 1903 in Copenhagen, the son of Prince Christian Frederik of Denmark. At birth he was named Christian Frederik, but was called Prince Frederik of Denmark.
When he was two his father was elected King of Finland, and Prince Frederik was given the name Kaarle. Kaarle was given a military education, serving in the Red Wars of 1919 - 1922. In 1921 he was invested as Crown Prince of Finland.
In 1922 he became a captain in the Helsinki guards, and in 1927 he became their colonel.