From the Memory Alpha article on [Mendon][1]:

> Mendon was a 24th century Benzite officer who, in 2365,
> participated in an Officer Exchange Program operated **between his
> government and the United Federation of Planets**.
> 
> Mendon specifically requested to be assigned to the USS Enterprise-D.
> **He came aboard at Starbase 179 with the rank of ensign.**

I would guess it's because his government was participating, that he was able to do the exchange as a Starfleet ensign.  He may well have a rank higher than that within his own government.

  [1]: http://en.memory-alpha.org/wiki/Mendon