Singleton's father was a barrister's clerk. Singleton was a stockbroker. At the time of the 1881 census Singleton was 37 years old and unmarried; at that time he had an an address in Staples' Inn, which is just across High Holborn from the alleys leading to Gray's Inn. On 7 March 1901, Singelton married Agnes Hiffernan, the sister of Whistler's former mistress, Joanna Hiffernan.
Singleton seems to have been a friend of long-standing, and certainly by 1876 he was giving Whistler advice and support, and was among the subscribers for a set of Venice etchings.
By 1879 they were so close that when Whistler actually went to Venice, he appears to have left his illegitimate son, Charles James Whistler Hanson (who may indeed have been Singleton's godson), under the care of Jo Hiffernan, in the Hiffernan/Singelton house in Thistle Grove, off the Old Brompton Road.
They remained friends until the end of JW's life, with JW visiting and dining with the Singletons (see #03569).
UK census 1881, from http://www.familysearch.org (accessed 2004).