Use a negative prefix argument to mark backwards, for example:

M-- M-@

Note that you can also mark a number of words forwards or backwards. For example, mark backwards four words:

M-- M-4 M-@

Use a negative prefix argument to mark backwards, for example:

M-- M-@

Note that you can also mark a number of words forwards or backwards. For example, mark backwards four words:

M-- M-4 M-@

Note that by default Emacs binds the prefix keys with a variety of modifiers. This makes it easier to specify a prefix when you are holding down some combination of modifiers for whatever command you are about to call. For example, I often use the following to mark a sexp backwards:

C-M--C-M-SPC.