Elizabeth Cary Agassiz was born into a wealthy family in Boston, Massachusetts, the second eldest of seven children. When she first met Louis Agassiz, he was still married to his wife, Cecile, who remained in Switzerland, and "Lizzie" Cary was still living at home with her parents and sister. They heard Agassiz lecture at the Lowell Institute in 1846, and soon met him through friends and became close friends. After Cecile died in 1848, he and Lizzie grew closer. Not wanting to appear too hasty or unfeeling toward Cecile, the couple did not become engaged until December of 1849, and were married on April 25, 1850. She never had children of her own, but became close with Louis's children from his first marriage.
Ever intrepid, Elizabeth managed and organized their research expeditions, kept records, and wrote for her husband while in the field. After Louis' death, Elizabeth published books on natural history, including A First Lesson in Natural History, Seaside Studies in Natural History, and A Journal in Brazil. She also wrote several pieces about her husband.
She was a co-founder of the Harvard Annex, a school for women connected to Harvard University, and was instrumental in the school's incorporation as Radcliffe College in 1894, where she was its first president. She was also one of the first female members of the American Philosophical Society.