Answer by Mico for vertical alignment of minipages in two columns in a longtable

You need to add the [t] positioning specifier not only to the minipage environments, but to both tabular environments as well.

%% preamble stuff omitted\begin{longtable}{|A|B|}    \fbox{    \begin{minipage}[t][6cm]{0.14\paperwidth}        \begin{tabular}[t]{|p{0.12\paperwidth}|} % <--- "[t]" is new            Left Table\\            left Table\\        \end{tabular}    \end{minipage}}&    \fbox{    \begin{minipage}[t][16cm]{0.69\paperwidth}        \begin{tabular}[t]{|p{0.66\paperwidth}|} % <--- "[t]" is new            Right Table\\            Right Table\\            Right Table\\            Right Table\\            Right Table\\            Right Table\\            Right Table\\            Right Table\\            Right Table\\        \end{tabular}    \end{minipage}}    \\\end{longtable}