Wishing to marry, but unable to support a family on the wages of a private secretary, Arnold sought the position of, and was appointed, in April 1851, one of Her Majesty's Inspectors of Schools. Two months later, he married Frances Lucy, daughter of Sir William Wightman, Justice of the Queen's Bench. The Arnolds had six children: Thomas (1852–1868); Trevenen William (1853–1872); Richard Penrose (1855–1908), an inspector of factories; Lucy Charlotte (1858–1934) who married Frederick W. Whitridge of New York, whom she had met during Arnold's American lecture tour; Eleanore Mary Caroline (1861–1936) married (1) Hon. Armine Wodehouse (MP) in 1889, (2) William Mansfield, 1st Viscount Sandhurst, in 1909; Basil Francis (1866–1868).
Arnold often described his duties as a school inspector as "drudgery," although "at other times he acknowledged the benefit of regular work." The inspectorship required him, at least at first, to travel constantly and across much of England. "Initially, Arnold was responsible for inspecting Nonconformist schools across a broad swath of central England. He spent many dreary hours during the 1850s in railway waiting-rooms and small-town hotels, and longer hours still in listening to children reciting their lessons and parents reciting their grievances. But that also meant that he, among the first generation of the railway age, travelled across more of England than any man of letters had ever done. Although his duties were later confined to a smaller area, Arnold knew the society of provincial England better than most of the metropolitan authors and politicians of the day."