There is every indication that George Johnston knew something of his father, which is not always the case with illegitimate children. So William was probably around the area where George grew up.
When George Johnston married in 1871 he named his father as
William Johnston, "agricultural labourer".Middlestots farm was (still is) just 3 miles from Duns. I cannot see any other place in the area with that same name.
The William Johnston at Middlestots in 1841 with his family, was still in the district in 1851 working as an agricultural labourer.
In 1851 he was working at Edrom Newton farm for Mr. George Carnegie. I think he is quite possibly the William Johnston you are looking for.
I haven’t found William in 1861 census.
By 1
871 census William Johnston is in Haldimand, Ontario, Canada working as a farm labourer for the McNeill family.
https://www.familysearch.org/ark:/61903/1:1:M4Q6-3RC?lang=enIn 1872 William Johnston (age 44)
married Martha DUNCAN (age 42) in Ontario.
https://www.familysearch.org/ark:/61903/1:1:FMKC-RY2?lang=enI don’t believe they had any children, at least none showing in 1881 census.
I can’t yet find a death for William, but when Martha died in 1905 she was a widow.
William’s parents and many of his siblings came to Canada before him.
The
1851 census for Haldimand, Ontario, Canada -
Adam Johnston, 62, farmer, and
wife Christian, 61, with children
Patrick 28, John 18 and Agness 21.
The
1871 census for Otonabee, Peterborough East, Ontario
Shows Adam and Christian Johnston farming, still with some of his adult children living with him.
https://www.familysearch.org/ark:/61903/1:1:M471-B72?lang=enAdam died in 1872 and Christian died in 1873.
If you have done a DNA test, I would check to see if you have any links with Johnstons in Canada.
[Perhaps a connection ALSO with the Johnston family of Middlestots Farm, Duns and New South Wales, Australia.
See this wikitree – don’t trust it, until you have checked all sources.
https://www.wikitree.com/wiki/Johnston-8403]