Clarissa Harlowe Barton was born on December 25, 1821, in Oxford, Massachusetts, to Stephen and Sarah Barton. She was the youngest of five children. Clara's father was a farmer and horse breeder, while her mother Sarah managed the household. The two later helped found the first Universalist Church in Oxford. When Clara was eleven, her brother David became her first patient after he fell from a rafter in their unfinished barn. Clara stayed at his side for three years and learned to administer all his medicines, including the "great, loathsome crawling leeches". As she continued to develop an interest in nursing, Clara may have drawn inspiration from stories of her great-aunt, Martha Ballard, who served the town of Hallowell (later Augusta), Maine, as a midwife for over three decades. Ballard helped deliver nearly one thousand infants between 1777 and 1812, and in many cases administered medical care in much the same way as a formally trained doctor of her era.

On his death bed, Clara's father gave her advice that she would later recall:

"As a patriot, he had me serve my country with all I had, even with my life if need be; as the daughter of an accepted Mason, he had me seek and comfort the afflicted everywhere, and as a Christian he charged me to honor God and love all kind. "The door that nobody else will go in at, seems always to open widely for me." said Clara Barton.

On April 20 1862, after the First Battle of Bull Run, Barton established the main agency to obtain and distribute most of the supplies to wounded soldiers. She was given a pass by General William Hammond to ride in army ambulances to provide comfort to the soldiers and nurse them back to health and lobbied the U.S. Army bureaucracy, at first without success, to bring her own medical supplies to the battlefields. Finally, in July 1862, she obtained permission to travel behind the lines, eventually reaching some of the grimmest battlefields of the war and serving during the Siege of Petersburg and Richmond, Virginia. In 1864 she was appointed by Union General Benjamin Butler as the "lady in charge" of the hospitals at the front of the Army of the James.

In 1865, President Abraham Lincoln placed Clara in charge of the search for the missing men of the Union Army. Around this time, a young soldier named Dorence Atwater came to her door. He had copied the list of the dead without being discovered by the Andersonville officials, and taken it with him through the lines when he was released from the prison. Having been afraid that the names of the dead would never get to the families, it was his intention to publish the list. He did accomplish this. His list of nearly 13,000 men was considered invaluable. When the war ended, Barton and Atwater were sent to Andersonville with 42 headboard carvers, and Barton gave credit to young Dorence for what came to be known as “The Atwater List” in her report of the venture. Dorence also has a report at the beginning of this list, still available through Andersonville National Historic Site in Georgia. Because of the work they did, they became known as the "Angels of Andersonville," according to a biography of Barton. She was also known as "The Angel of the Battlefield". Her work in Andersonville is displayed in the book, Numbering All the Bones, by Ann Rinaldi. This experience launched her on a nationwide campaign to identify all soldiers missing during the Civil War. She published lists of names in newspapers and exchanged letters with soldiers’ families.

Barton then achieved widespread recognition by delivering lectures around the country about her war experiences. She met Susan B. Anthony and began a long association with the woman's suffrage movement. She also became acquainted with Frederick Douglass and became an activist for black civil rights, or an abolitionist.

When Clara Barton returned to the United States, she inaugurated a movement to gain recognition for the International Committee of the Red Cross by the United States government. When she began work on this project in 1873, most Americans thought the U.S. would never again face a calamity like the Civil War, but Barton finally succeeded during the administration of President James Garfield, using the argument that the new American Red Cross could respond to crises other than war. Barton naturally became President of the American branch of the society, which was founded on May 21, 1881 in Dansville, N.Y.

This changed with the advent of the Spanish-American War during which it aided refugees and prisoners of war. In 1896, responding to the humanitarian crisis in the Ottoman Empire in the aftermath of the Hamidian Massacres, Barton sailed to Istanbul and after long negotiations with Abdul Hamid II, opened the first American International Red Cross headquarters in the heart of Beijing, China. Barton herself traveled along with five other Red Cross expeditions to the Armenian provinces in the spring of 1896. Barton also worked in hospitals in Cuba in 1898 at the age of seventy-seven. Barton's last field operation as President of the American Red Cross was the relief effort for the victims of the Galveston hurricane of September 1900. The operation established an orphanage for children of the 6,000 dead, helped to acquire lumber for rebuilding houses, and teamed with the New York World newspaper to accept contributions for the relief effort. As criticism arose of her management of the American Red Cross, plus her advancing age, Barton resigned as president in 1904, at the age of 83. On April 12, 1912 at the age of 90 she died in Glen Echo, Maryland, with all her friends by her side.

While various authorities have called Barton a “Deist-Unitarian" or freethinker or deist; in a 1905 letter to Mrs. Norman Thrasher, she called herself an "Universalist”. The term was used then to describe those believing the Universalist Church doctrine, which was also the Church her parents belonged to.

Clara Barton Birthplace Museum in North Oxford, Massachusetts, is operated as part of the Barton Center for Diabetes Education, a humanitarian project established in her honor to educate and support children with diabetes and their families.

In 1975, Clara Barton National Historic Site was established as a unit of the National Park Service at Barton's Glen Echo, Maryland, home, where she spent the last 15 years of her life. One of the first National Historic Sites dedicated to the accomplishments of a woman, it preserves the early history of the American Red Cross, since the home also served as an early headquarters of the organization. The National Park Service has restored eleven rooms, including the Red Cross offices, the parlors and Barton's bedroom. Visitors to Clara Barton National Historic Site can gain a sense of how Barton lived and worked.