Mahjoub deservedly prevailed over opponents from Russia, France and the host nation in his over 100-kilogram weight class contests to march into the final.
The Iranian sportsman, however, could not overcome his strong Georgian rival Guram Tushishvili in the title clash, and finished as the vice champion.
Tbilisi Grand Prix 2018 started on March 30, and finished on April 1.
The tournament attracted 302 judokas from 38 nations, including Australia, Brazil, Canada, France, Georgia, Germany, the Islamic Republic of Iran, Poland, Russia, Switzerland, Ukraine and the United States.