Samuel Clemens does appear in the following censuses that are available at
ancestry.com:

1850 - Hannibal, Missouri
1860 - St. Louis, Missouri
1870 - Buffalo, New York
1880 - Hartford, CT
1900 - Hartford, CT
1910 - Redding, CT

The Redding census sheet is interesting because the census taker has dated
it one day after Clemens's death, April 22.  However, user comments on
ancestry regarding this sheet indicate the actual census was taken on April
15, 1910.

If you do not have a full subscription to ancestry.com it may be difficult
to zero in on these sheets.  However, if you are a subscriber, there are a
list of SUGGESTED RECORDS for each individual that lead directly to the
other census sheets.  His name has often been misspelled by transcribers
and other researchers have provided corrected spellings and
identifications. There are a number of amusing transcriber errors related
to some of these census sheets.  For example, the 1870 census sheet has
Clemens's occupation as "clerk in dry goods" rather than "Prop. daily
paper". The 1910 census sheet has his occupation transcribed as "Waiter"
"hardware st" rather than "Writer" "Humorist."

As to the 1840 census -- Samuel Clemens is not listed by name, but his
father John Marshall Clemens and household members are in the Mason
Township, Marion County, Missouri census.  As another Forum member, Bob
Stewart, recently pointed out to me, there is only one slave listed in the
family household -- a female between the ages of 10 - 23.

Barb