Walter Mercer was the son of George Mercer and Janet Mercer, born in approximately 1806 in Edinburgh, Scotland. Like his father, he worked at a lace maker and hosier. He married Helen Mackenzie in 22 October 1838. They had a number of children together, including Helen, Jessie, George, and Anne. He died in the 1880s.

Janet Morison was born on 23 July 1781 in Edinburgh, Scotland, the daughter of Hugh Morison and Jean Flint. She married George Mercer on 25 December 1805, and together they had five children: Walter, Jane, Marion, Jessie, and Margaret. She died on 21 September 1850.