The daughter of Thomas Strickland and Elizabeth (born Homer) of Reydon Hall, Suffolk, Agnes and her elder sister Elizabeth were educated by their father as if they were boys. Agnes was born at Rotherhithe in Surrey, (she was christened in Rotherhithe on 18th August 1796) where her father was employed as a manager of the Greenland Dock. The family moved to Reydon Hall in 1808. Her siblings were Elizabeth; Sarah; Jane Margaret, Catharine Parr, Susanna Moodie (1803–1885) and Samuel Strickland. All of the children except Sarah eventually became writers.
She began her literary career with a poem, Worcester Field, followed by The Seven Ages of Woman and Demetrius. Abandoning poetry, she produced Historical Tales of Illustrious British Children (1833), The Pilgrims of Walsingham (1835) and Tales and Stories from History (1836). Her chief works, however, are Lives of the Queens of England from the Norman Conquest, and Lives of the Queens of Scotland, and English Princesses, etc.. (8 vols., 1850–1859), Lives of the Bachelor Kings of England (1861), and Letters of Mary Queen of Scots, in some of which she was assisted by her sister Elizabeth. Strickland's researches were laborious and conscientious, and she remains a useful source. Her style is not as objective as most modern historians, but gives a valuable insight into the mores of her own time.
Most of the Strickland sisters' historical research and writing was done by Elizabeth. Elizabeth however refused all publicity and Agnes was put forward as author. Their biographical works are fine representations of the larger body of biographies written by Victorian women, a significant subset of Victorian biography with unique characteristics, including the focus on female subjects and inclusion of information that was more "social" in nature, such as dress, manners, and diet.
Two of her sisters, Susanna Moodie and Catharine Parr Traill, became known for their works about pioneer life in early Canada, where they both emigrated with their husbands in 1832.