Diallo made just five Ligue 1 appearances last season paired with six runouts in Monaco's domestic and continental cup runs. Leonardo Jardim preferred a centre-back pairing of Kamil Glik and Jemerson, with Andrea Raggi the first off the bench.
The Tours-born 21-year-old also made one start at left-back, though even with a potential exit for first-choice Benjamin Mendy, January signing Jorge still appeared to be Jardim's pick.
"Abdou Diallo is a strong player and an intelligent two-way player who has aroused a lot of enthusiasm on the European market," Mainz sporting director Rouven Schroder said.
"In addition, he is a very purposeful young man with good character. We are delighted that he has decided with full conviction to go to the next step at Mainz 05."