In this paper, we study and investigate unified and extended fractional integral operator involving the multivariable $I$-function defined by Prasad, Raizada's generalized polynomial set and general class of multivariable polynomials. During the present study, we derive five theorems pertaining to Mellin transforms of these operators. Furthermore, on account of the general nature of the functions involved herein, many known and (presumably) new fractional integral operators involved simpler functions can be obtained. We also give the special case concerning the multivariable $H$-function.