Lady Juliana was born in 1729 at Easton Neston, Northamptonshire, the fourth daughter of Thomas Fermor, 1st Earl of Pomfret and Henrietta Louisa Jeffreys.

Thomas, formerly a Quaker, attended Anglican church services regularly after their marriage, though he did not take part in the sacrament of Communion.

She corresponded with Governor John Penn (her husband's nephew) and other colonial officials, including discussing maps and other materials of administration.

"[7][8] Penn and her co-executor William Baker took an active interest in the survey of Susquehanna Land Company holdings in the Wyoming Valley and wrote to James Tilghman expressing their hopes for a favorable outcome.

[9] The Reverend Jacob Duché wrote to Benjamin Franklin about visiting "my most Amiable Friend Lady Juliana Penn," during an official trip to England in 1783.