Gustaf Lennart Nicolaus Paul was born in Stockholm in May 1909, and was the son of Prince Wilhelm and Maria Pavlovna. Prince Lennart was Duke of Småland until 1932, and Count of Wisborg from 1951.
After his parents' divorce, Lennart's grandmother Queen Viktoria took over his upbringing. When Lennart Bernadotte was a teenager, he moved in with his father at Stenhammar Palace. The Prince completed his school exams in 1927 and became a Reserve Lieutenant in 1930.
Before her death in 1930, his grandmother had left her Lake Constance castle and estate, Mainau, to the prince and he took over its management in 1932. That same year, he lost the title of Prince and his duchy when he married Karin Nissvandt. He was then Mr Bernadotte until 1951, when he was granted the title Count Bernadotte of Wisborg by the Grand Duchess Charlotte of Luxembourg.
His marriage to Karin Nissvandt was dissolved in December 1971. On 29 April 1972 he married Sonja Haunz.
Until his death in 2004, Lennart Bernadotte owned and managed Mainau Castle. The castle's grounds are known as a floral paradise.