I searched all the years for which Samuel Clemens should (or might) show up in the U.S. census, from 1840 to 1910 inclusive, and he only appears in two: 1850, and 1880.
I realize that the 1910 census probably took place following his death, and in 1900 he may have been in Europe, but it's a little surprising he wasn't located the other years.
The 1880 census sheet is interesting in that it shows everyone in the household (including servants).
I saved a copy of it (from familysearch.org) as an image, if anybody wants it. I don't think this forum accepts attachments, so I would need your email address. I also saved the "human legible" details for the 1850 and 1880 censuses, if any want those.
The 1850 census sheet is not legible. - B. Clay Shannon