Carcinoembryonic antigen (CEA) is an important oncomarker for the detection of breast cancer. For ultra-sensitive sensing of CEA with great specificity and accuracy, an innovative and reliable electrochemical immunosensor was developed using various nano-hybrids. A glassy carbon electrode (GC) was modified with thiolated graphene oxide (T-GO) to elevate the active surface area of the electrode. The streptavidin-coated gold nanoparticles (AuNPs) were used to increase the conductivity of the sensing area as well as the loading capacity of the biotinylated monoclonal antibody (mAb). A sandwich-on approach was developed to reach a low limit of detection (LOD). The biotinylated mAb, streptavidin coated silver nanoparticles (AgNPs) and horseradish peroxidase (HRP), altogether, formed the signaling probe of the proposed immunosensor. The electrochemical signal was significantly enhanced in the presence of hydroquinone (HQ) and hydrogen peroxide (H2O2). Under the optimized conditions, the proposed immunosensor presented an excellent performance in a linear range of 100 fg/mL to 5 pg/mL with a low detection limit of 75 fg/mL. The engineered immunosensor displayed excellent specificity for the detection of CEA even in the real human serum, upon which it is proposed for the early detection and monitoring of CEA in the clinic.