The Collaborative International Dictionary of English v.0.48:

Mandator \Man*da"tor\, n. [L.]
   1. A director; one who gives a mandate or order. --Ayliffe.
      [1913 Webster]

   2. (Rom. Law) The person who employs another to perform a
      mandate. --Bouvier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mandator
    n 1: an authority who issues a mandate

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MANDATOR, contracts. The person employing another to perform a mandate.
Story on Bailm. Sec. 138; 1 Brown, Civ. Law, 382; Halif. Anal. Civ. Law, 70.