Sangwan was born in 1900 into a goldsmith family in the Anongkharam Temple community in Thonburi. Her mother taught her and her brother to read and write, and encouraged her love of reading. Sangwan was a precocious reader. Book rental shops were her refuge, and the owner of one local book rental shop liked the bright young girl and waived the rental fees for her. This opened her mind to the vast world of knowledge and learning, a quality that she retained for the rest of her life.
After losing her father and mother at a young age, she was presented to the court of Princess Valaya Alongkorn, who allowed her to continue her schooling at Satri Witthaya School. At the age of 13, she was accepted into the School of Nursing and Midwifery, Siriraj Hospital. Although she was two years under the minimum requirement age, she was accepted because of her intellectual maturity. She went on to win a nursing scholarship to the US. This was how she met her future husband, Prince Mahidol – son of King Chulalongkorn, Rama V, and Queen Savang Vadhana – who was then a medical student at Harvard University. The couple were married in 1920.