The Mazda Millenia was originally planned as the smaller of two luxury cars for Mazda's luxury brand, Amati. As Mazda's dwindling finances no longer permitted the launch of the Amati brand, the Millenia was launched in the autumn of 1993 in Europe (as the Xedos 9) and Japan (as the Eunos 800) under the Mazda brand. The car was launched in the US in 1995 as the Millenia, and replaced the 929 as Mazda's flagship sedan. The Millenia was facelifted for the 2000 through 2002 model years. There was no model link to the other Mazda marque Ẽfini.

Having been developed for a separate audience from typical Mazda customers, the Millenia boasted a myriad of finer details. It was engineered to far greater levels of perceived quality than existing Mazda cars, such as interior plastic, panel gap and thicker paint coating. The Millenia/Eunos 800/Xedos 9 was assembled along with the smaller Eunos 500/Xedos 6 in a new production line, presumably set up for Amati cars.

The Millenia does not have a predecessor or replacement in the Mazda product line. It appears to have received a brand-new platform, although suspension at both ends resembled that of the 1991 Mazda Sentia. It is the only production car in the world to employ a Miller cycle engine. Yaw-sensitive 4-wheel steering was available as an option in Japan; Mazda claimed that with this feature, the Millenia was capable of passing the elk test at speeds comparable to the BMW 850i, surpassing the Z32 Nissan 300ZX.

In 1997 the Japanese Eunos 800 was also renamed the Millenia as Mazda reorganized its Eunos dealership network. This name change was accompanied by a significant facelift that included some cost-saving measures. For instance, the hood was downgraded from aluminum to steel.

In the USA, the Millenia was available with or without the Miller Cycle engine. Vehicles with the supercharged engine had a badge on the back that read "Millenia S" for supercharged.