In 1857 he moved back to Hackney, becoming headmaster at the St. John's Foundation School and Assistant Reader at the Chapel of the Foundling Hospital in nearby Brunswick Square.
In 1865 Lewis Mercier suddenly found it necessary to borrow £250[15] secured by a bond from Lord Leigh of Stoneleigh (Warwickshire), the wealthiest landowner in England, at the rate of 12% per annum.
By 1870, in ill health and unable to repay his debt when it came due, he was forced to seek extra funds by offering to perform translating tasks for the publishers Sampson Low, Marston, Searle, and Rivington, who were in the process of printing a religious book of his.
A linguist of sorts, conversant with older French dialects, possibly fluent in multiple languages, he worked in his spare time with his assistant Eleanor Elizabeth King (1838 — ??)
Mercier's shaky grasp of contemporary French idioms and limited knowledge of his era's science and technology led him into many foolish translating errors.
In view of his desperate financial situation it has been conjectured that the deletions could have been dictated by his editors at Sampson Low; however no hard evidence has been found to support this surmise.
[17] Mercier was forced to resign[18] his position at the Foundling Hospital by the Governing Board in early 1873 after troubles arose over his supervision of the schools, and he died on Tuesday, 2 November 1875,[19] the date his semi-annual payment of £15 to Lord Leigh was due.
In 1874 the Polish expatriate, revolutionary, member of the Paris Commune, American war correspondent, and author Stephan Poles (1847–1875)[20] published a pamphlet denouncing in strong language the secretary and chief librarian of the British Museum, John Winter Jones.
Mercier also translated The Wreck of the Hansa (The German Arctic expedition of 1869-70), as well as publishing several religious works and instructional materials for teachers of Greek and Latin listed below: