Gardner was born on November 5, 1865, in Boston, Massachusetts, to Joseph Peabody Gardner and Harriet Sears Amory.
He was the descendant of Thomas Gardner (planter) and nephew of John Lowell "Jack" Gardner II whose wife was Isabella Stewart Gardner.
Jack and Isabella 'adopted' Augustus and his two brothers (Joseph and William) after the death of their father in 1875.
Their mother had died in 1865.
He graduated from Harvard University in 1886.
He studied law at Harvard Law School, but never practiced, instead devoting himself to the management of his estate.

On June 14, 1892, Gardner married Constance Lodge, daughter of Henry Cabot Lodge, at Saint Anne's Church, Nahant, Massachusetts.
After his death, Constance married Major General Charles Clarence Williams, who served as U.S. Army Chief of Ordnance from July 1918 until April 1930.
Gardner was born on November 5, 1865, in Boston, Massachusetts, to Joseph Peabody Gardner and Harriet Sears Amory. He was the descendant of Thomas Gardner (planter) and nephew of John Lowell "Jack" Gardner II whose wife was Isabella Stewart Gardner. Jack and Isabella 'adopted' Augustus and his two brothers (Joseph and William) after the death of their father in 1875. Their mother had died in 1865. He graduated from Harvard University in 1886. He studied law at Harvard Law School, but never practiced, instead devoting himself to the management of his estate.
On June 14, 1892, Gardner married Constance Lodge, daughter of Henry Cabot Lodge, at Saint Anne's Church, Nahant, Massachusetts. After his death, Constance married Major General Charles Clarence Williams, who served as U.S. Army Chief of Ordnance from July 1918 until April 1930.
Augustus Peabody Gardner