Over the past few years, lead-halide perovskites (LHPs), both in the form of bulk thin films and colloidal nanocrystals (NCs), have revolutionized the field of optoelectronics, emerging at the forefront of next-generation optoelectronics. The power conversion efficiency (PCE) of halide perovskite solar cells has increased from 10% to over 25% over a short period of time and is very close to the theoretical limit (30%). At the same time, the external quantum efficiency (EQE) of perovskite LEDs has surpassed 23% and 20% for green and red emitters, respectively. Despite great progress in device efficiencies, the photoactive phase instability of perovskites is one of the major concerns for the long-term stability of the devices and is limiting their transition to commercialization. In this regard, researchers have found that the phase stability of LHPs and the reproducibility of the device performance can be improved by A-site cation alloying with two or more species, these are named mixed cation (double, triple, or quadruple) halide perovskites. This review provides a state-of-the-art overview of different types of mixed A-site cation bulk perovskite thin films and colloidal NCs reported in the literature, along with a discussion of their synthesis, properties, and progress in solar cells and LEDs.