Barnard grew up in Beaufort West, Cape Province, Union of South Africa. His father, Adam Barnard, was a minister in the Dutch Reformed Church. One of his four brothers, Abraham, was a "blue baby" who died of a heart problem at the age of three (Barnard would later guess that it was tetralogy of Fallot). The family also experienced the loss of a daughter who was stillborn and who had been the fraternal twin of Barnard's older brother Johannes, who was twelve years older than Chris.

Barnard performed the world's first human-to-human heart transplant operation in the early morning hours of Sunday 3 December 1967. Louis Washkansky, a 54-year-old grocer who was suffering from diabetes and incurable heart disease, was the patient. Barnard was assisted by his brother Marius Barnard, as well as a team of thirty persons. The operation lasted approximately five hours.  Barnard stated to Washkansky and his wife Ann Washkansky that the transplant had an 80% chance of success. This has been criticised by the ethicists Peter Singer and Helga Kuhse as making claims for chances of success to the patient and family which were "unfounded" and "misleading".  Barnard later wrote, "For a dying man it is not a difficult decision because he knows he is at the end. If a lion chases you to the bank of a river filled with crocodiles, you will leap into the water, convinced you have a chance to swim to the other side." The donor heart came from a young woman, Denise Darvall, who had been rendered brain dead in an accident on 2 December 1967, while crossing a street in Cape Town. On examination at Groote Schuur hospital, Darvall had two serious fractures in her skull, with no electrical activity in her brain detected, and no sign of pain when ice water was poured into her ear. Coert Venter and Bertie Bosman requested permission from Darvall's father for Denise's heart to be used in the transplant attempt. The afternoon before his first transplant, Barnard dozed at his home while listening to music. When he awoke, he decided to modify Shumway and Lower's technique. Instead of cutting straight across the back of the atrial chambers of the donor heart, he would avoid damage to the septum and instead cut two small holes for the venae cavae and pulmonary veins. Prior to the transplant, rather than wait for Darvall's heart to stop beating, at his brother Marius Barnard's urging, Christiaan had injected potassium into her heart to paralyse it and render her technically dead by the whole-body standard. Twenty years later, Marius Barnard recounted, "Chris stood there for a few moments, watching, then stood back and said, 'It works.'"  Washkansky survived the operation and lived for 18 days, having succumbed to pneumonia as he was taking immunosuppressive drugs.

Who was the recipient of the heart?
Louis Washkansky, a 54-year-old grocer who was suffering from diabetes and incurable heart disease, was the patient.