[3][4] She had two siblings; her older sister, Ellen Mary (1840-1906) and her younger brother, Aubrey St. John (1843-1923).

At the age of 25, partly for health reasons[8] together with her elder sister Ellen, she went to Italy where she stayed until 1877, chiefly at Florence, studying science, languages, and other subjects that would be useful in their later lives.

[2] During her career she wrote reviews of many books, including some written in French, German, Greek, or Italian.

[11] In the autumn of 1890, Clerke and her brother Aubrey were founding members of the British Astronomical Association.

In 1903, with Margaret Lindsay Huggins, she was elected an honorary member of the Royal Astronomical Society, a rank previously held only by three other women, Caroline Herschel and Mary Somerville (in 1835), and Anne Sheepshanks (in 1862).

The performance included songs specially composed by Jessie Kennedy and by Tess Leak, using words of Agnes and her sister, Ellen, and a cello trio, the Agnes Clerke Cello Trio, composed by Diana Llewellyn.

Her articles in the ninth edition (1875–89) of the Britannica included Galileo Galilei, Alexander von Humboldt, Johannes Kepler, Antoine Lavoisier and the zodiac.