Miniature diagnostic devices combined with powerful smartphone processors have revolutionized healthcare diagnostics by bringing it to our homes, where it was previously confined to laboratories. Smartphone-assisted glucose monitoring systems like iBGStar and iHealth exemplify this by incorporating glucometers in smartphones for data display, analysis and tracking of glucose levels. Inbuilt sensors in smartphones like cameras are being utilized as analytical tools in microfluidic and colourimetric assays for disease diagnosis, where the RGB (red, green blue) intensities can be quantified to determine the severity of the disease. During the COVID-19 pandemic, research in this field rapidly accelerated, as observed by the rapid development and commercial availability of many smartphone-based sensors like the iHealth and Ellume COVID-19 antigen detection kits and application software like the Arogya Setu app for contact tracing using Bluetooth assistance within small distances. AI-assisted Chabot and telehealth platforms further helped patients to remotely access doctors in limited resource settings, while the wearable sensors in smartwatches helped in personalized monitoring of physiological parameters. This promoted isolation of patients, which was a mandate for containment of the disease, indicating their significance in decentralised health management. Wearable sensors offer futuristic development using implantable biosensors, wherein tiny biosensor chips are placed under the skin or in the body to monitor various biological and physicochemical parameters, ranging from diabetes, cardiac movements, and brain activity to cancer biomarkers, in patients for precision and personalized healthcare. The review analyzes the sensing technologies employed in smartphone biosensors, further addressing the strengths and limitations of their integration within point-of-care devices. It concludes by focusing on the future direction of research and development and the potential of smartphone biosensors for personalized and decentralized health management.