In the 1851 census there�s a �Thos Johnson� listed as a servant to the Barlow family; his age is given as 17; the head of the household is Josh Barlow, aged 52 and born in Derbyshire. The family is resident in Barlow, but the image of the census is very difficult to read.
There are several people called �William Johnson� born in the mid to late 1830s in Sheffield in the same census; the most likely has his year of birth given as 1839 and is living in Brightside with his parents George and Mary and his older sister Ann. They are living at 39 Walker Street, Brightside Bierlow, Sheffield.
William Johnson, aged 2, and Thomas Johnson, aged 8, are listed together in the 1841 census; that census doesn�t show family
Relationships, but the woman I guess to be their mother is called Mary.
There is a William Johnson and wife Elizabeth in the 1881 census in Sheffield living at 171 Tinsley Road; there is a �Wm Johnston� aged 42, in the same census, living in Headford St, Sheffield, whose wife is called Eliza and whose children are George, John, Eliza, Fred, William, Emma and Florence. This family appears in the 1871 census with their surname spelt �Johnson�.
If a family disappears from one census to the next (and your are searching on ancestry) it�s always worth putting in as many details as you can, without the surname � this will help you find mistranscriptions.