Katherine Charlotte Baillie married Bertram Ashburnham, 4th Earl of Ashburnham, in 1840. He was the fourth son of George Ashburnham, 3rd Earl of Ashburnham. The 4th Earl was a bibliophile who amassed an important collection of printed books and manuscripts and was known as "one of the great collectors of the nineteenth century". They had four daughters and seven sons.