Organic mixed ionic-electronic conductors (OMIECs) are touted as a highly promising sub-class of organic electronics that see application in organic energy storage, where global scale implementation is envisioned, as well as bioelectronics, where biocompatibility is an additional key requirement. Therefore, the ongoing development of new OMIECs should not just focus on developing materials of high performance in target applications, but also place increasing emphasis on developing materials of low environmental footprint, in line with the future need for sustainable electronics. To empower this direction of OMIEC research, the following review first explores the emerging applications of OMIECs in organic electrochemical transistors (OECTs) and biosensing, signal processing and neuromorphic computing, as well as organic energy storage, to distil the key materials characteristics required for high performance in each target application. A summary of the three different categories of OMIECs, which include those based on small molecules, conjugated polymers and 2D/3D covalent-organic frameworks is also provided, to highlight the key characteristics of each OMIEC and suitability for specific applications. Finally, strategies that enable the low environmental footprint synthesis and materials design diversification of OMIECs are discussed, which encompass the deployment of more environmentally benign cross-coupling and metal-free polymerisations, as well as post-synthetic modification.