Herbert is said to have had a five-year affair in the early 1840s, with author and social reformer Caroline Norton, but they separated since she was unable to obtain a divorce.

Lady Herbert of Lea became a Roman Catholic convert at Palermo in 1866, practicing as an "ardent Ultramontane", under the influence of her intimate friend, Cardinal Manning.

[7] She disliked "of Lea" as an addition to her title, and never used it, becoming known as "Lady Lightning" for her efficiency and ardour working for Catholic charities and interests.

She worked in partnership with Cardinal Vaughan for St Joseph's Foreign Missionary College, Mill Hill Park, London, which was opened in 1869.

Disraeli described her as: She was the daughter of a Protestant house, but, during a residence at Rome after her marriage, she had reverted to the ancient faith, which she professed with the enthusiastic convictions of a convert.

Besides these she wrote several stories, some of them autobiographical, articles (many contributed to the Dublin Review), and a number of biographies and biographical essays, mostly of religious figures, which were translated or paraphrased from French originals.