The mirrored interferometric aperture synthesis (MIAS) is a promising technique for high-resolution observation in microwave radiometry. In this article, we present a new source localization method based on a novel hybrid coarray that can be constructed from the 1-D MIAS. The new method, named <italic>MA-SS</italic> method, employs the spatial smoothing (SS) technique on the hybrid coarray of mirrored array (MA) in the MIAS. We show that it has the ability to resolve more sources than physical sensors. The theoretical analysis gives an upper bound on degrees of freedom (DOF) of <inline-formula> <tex-math notation="LaTeX">$O(N^{2})$ </tex-math></inline-formula> order using <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> physical sensors. Simulation and experiment results demonstrate that, compared with the discrete cosine transform (DCT) approach commonly utilized in the MIAS, the presented MA-SS method shows the superiorities on spatial resolution, sidelobe reduction, localization accuracy, and detection performance.