Octavius Thomas Clark was the sixth surviving child of animal painter James Clark and Elizabeth (nee London), although his name suggests he may have been the eighth born. He was born 21 December 1850 at 28 Parr Street, Hoxton, which was then part of Shoreditch. He appears on his family's census records of 1851 and 1861 as simply "Thomas Clark".

In 1874, when he was 23, Octavius married Ellen Louisa Maidment at Christ Church, Hoxton. At first it seemed that Octavius was not to follow the family tradition of a career in art, as his elder brothers Samuel, Albert and Charles had done. On his marriage certificate his occupation is given as "mercantile clerk", and as "clerk" on the baptism record of his first child, Louis E...