I have a John T. Mason (6 Feb 1870 - 24 Dec 1901) who is the son of Thomas Mason and Emma Jane Harmon. Thomas and Emma lived in the Round Grove area of Mt. Pleasant Twp or across the township line into Hopkins Township in 1870. They later moved north to Clyde Township and were found there in 1880 and 1900 census info. I'm not sure if this is the John you are looking for since he would have been only a young boy for the dates you have mentioned.

I have also found another John Mason in the 1850 census for Whiteside Co. You might try looking at that census to see if that is the John you need. Also try the Whiteside board - there are many helpful people with many resources available.

Tracy