She was the daughter of John Christopher Hiffernan, a clerk or writing master. She was the sister of Whistler's's model and mistress, Joanna Hiffernan. Agnes was the mistress and later the wife of the accountant Charles John Singleton. Both Joanna Hiffernan and JW's son, Charles J. W. Hanson, stayed at times in the Singleton's house at Thistle Grove in Chelsea. At the time of their marriage she was living at 91 Ridgmount Gardens, St Pancras.
She posed for JW's etching, Agnes (K.134). She was among the mourners at Whistler's's funeral.
UK Census 1871; UK census 1881 from http://www.familysearch.org (accessed 2004); marriage certificate, 1901, Family Records Office, LondonMorning Post, London, 23 July 1903.