A comprehensive overview of antenna evolution for mobile devices is initially presented. A detailed list of desired specifications for future mobile devices is illustrated with adequate examples with reference to the form factor of modern commercial smartphones. The desired specifications are specifically listed for 4G LTE (Long term Evolution) and mmWave 5G antennas and its coexistence. These desired specifications are tailored for modern commercial ultra-thin smartphones. Feasibilities of up-scaling of 4G antennas and down-scaling of 5G antennas are presented with the necessary design justifications. Co-designing 4G LTE and mmWave 5G antennas with optimal characteristics is the appropriate method to achieve backward compatibility. Design examples for the co-design of 4G-LTE and 5G antennas are also presented which meet the desired specifications and are compliant with modern mobile devices. Some design considerations are also introduced for future 4G–5G co-designed antennas integrated inside a smartphone.