Hello GRWL,
If you are certain your William Mills, is the son of John Mills, and Isabella Sharp, then this William Mills, born 23/July/1865. in Crieff, Perth, is living at 67 Cedar Street, Glasgow, Lanark, in 1881. with his parents and siblings.
William Mills, married Isabella Sharp, 13/July/1855. Perth. This couple had nine children that i can see.
Regards,
Jonn.