Agnes was the ninth of fourteen children, and the third daughter of her father, the Reverend Baden Powell, who was the Savilian Professor of Geometry at the University of Oxford. Among her brothers were Warington Baden-Powell, Sir George Baden-Powell, Frank Baden-Powell, and Baden Baden-Powell. Her mother, Henrietta Grace Smyth, was the third wife of Rev. Baden Powell. Henrietta Grace was a gifted musician and artist, a daughter of Admiral William Henry Smyth and Annarella Warington.
When Agnes was only two years of age, the Reverend Baden Powell died. To honour him after his death, Agnes's mother Henrietta added Baden to their surname and that branch of the family has since been known as Baden-Powell.
Baden's death left the family under the firm control of Henrietta, who was determined to instill in her children a desire to succeed. Agnes' brother, Robert, has been quoted as saying, "The whole secret of my getting on lay with my mother."
Agnes went on to become an accomplished musician, playing the organ, piano and violin. Her varied interests included natural history and astronomy, and she kept bees, birds and butterflies in her home.
In April 1901 Agnes became engaged to Sir William Bisset Berry, the Speaker of the South African Parliament, but they did not marry. Some years later she became very friendly with Guglielmo Marconi.
With her brother Baden Fletcher Smyth Baden-Powell Agnes made aeronautical balloons, working the silk for the envelope, and they made many flights together. Later she helped him with aeroplane-building. Agnes was an honorary companion of the Royal Aeronautical Society from 1938.
She was for some years president of the Westminster Division of the Red Cross, and worked for the League of Mercy and for Queen Mary's Needlework Guild.
Following the creation of the Boy Scout Association, Robert Baden-Powell organised a gathering of Scouts at the Crystal Palace in London in 1909. He was upset to see amongst the many Girl Scouts gathered a small group without tickets who had gatecrashed the event.
Popular opinion at this time was against mixed activities for girls, growing pressure persuaded Robert Baden-Powell to consider setting up a separate organisation for the Girl Scouts, and having been turned down by first aid societies, he approached his sister, Agnes, who reluctantly agreed to take on the organising of the new sister group, Girl Guides. Agnes Baden-Powell's character was useful in counteracting negative opinions of the new Girl Guides. A friend wrote of her:
Anyone who had come into touch with her gentle influence, her interest in all womanly arts, and her love of birds, insects, and flowers, would scoff at the idea of her being the president of a sort of Amazon Cadet Corps.
In 1909, Robert Baden-Powell published "Pamphlet A: Baden-Powell Girl Guides, a Suggestion for Character Training for Girls" and "Pamphlet B: Baden-Powell Girl Guides, a Suggestion for Character Training for Girls". These were precursors to the handbook.
By April 1910 there were 6,000 young girls registered as Girl Guides. In 1912, Agnes brought about the formation of the 1st Lone Company and was the de facto president of The Girl Guide Association.
During this time, Agnes set to create the Guides' first handbook. Entitled The Handbook for the Girl Guides or How Girls Can Help to Build Up the Empire, and published in 1912, it was a reworking of the Scouting for Boys book written by Robert several years earlier. The Girl Guide movement was given official recognition in 1915. In 1916 the new county commissioners voted Olave Baden-Powell into the new post of chief guide, and Agnes was offered the honorary post of president which she reluctantly accepted.
In 1917, following pressure, Agnes resigned from the presidency in favour of Princess Mary, who was also a keen supporter of the Girl Guides.
Agnes continued in her role as vice-president until her death in 1945.