Cancer is one of the most extensive diseases with the highest mortality rate, accounting for almost 10 million deaths in 2020. The most common cancers are breast, lung, colon and rectum and prostate cancers. Of these, lung cancer, accounted for about 1.8 million of all cancer deaths (25%) in 2020. Detection of cancer relies on presence of biomarkers such as DNA molecules, proteins and metabolites released by cancerous cells into the circulation. Carcinoembryonic antigen (CEA) is one of the biomarkers that has been used for the detection of lung cancer. However, CEA is not specific to lung cancer since it is also manifested in gastric cancer, pancreatic cancer, colorectal cancer, and breast cancer. Recently, v-YES1 Yamaguchi sarcoma viral oncogene homolog 1 (YES1) was described as a specific biomarker for lung cancer. The detection of both CEA and YES1 would give more precise and authentic information for detecting lung cancer. This is because detection of a single tumor marker usually limits the precision in tumor diagnosis, due to the fact that several cancers have more than one marker linked with their prevalence. Whereas traditional methods have been used for the detection of CEA, electrochemical immunosensors have attracted considerable attention owing to their profound advantages, including fast response, miniaturization, high selectivity, low sample requirements and magnificent sensitivity. The fabrication of a multiplex and simultaneous immunosensor is met with challenge of preparation of distinguishable immunoprobes with different redox activities. This can be addressed by incorporation of electroactive Nano metals into the sensing platform. In this study, gold nanoparticles were used for the fabrication of an ultrasensitive sandwich electrochemical multiplex immunosensor for simultaneous detection of CEA and YES1. Under optimized conditions, the electrochemical immunosensor detection limit for YES1 and CEA was found to be 0.0022 and 0.0034 ng/mL respectively within a linear range of 0.1–50 ng/mL. The proposed immunosensor proved to be stable for up to 2 weeks and had negligible cross reactivity towards various interfering compounds in human plasma. This study reports that gold nanoparticles can be bio synthesized using shade dried Mangifera indica leaves extract. The bio-synthesized gold nanoparticles coupled with thiolated protein G can be used for fabrication of a multiplex immunosensor for detection of CEA and YES1. The proposed immunosensor can provide a new approach for early diagnosis of circulating cancer biomarkers and holds great promise for application in clinical diagnosis.