This article presents a vertically polarized (VP) endfire leaky-wave antenna (LWA) on a low-profile substrate by loading a series of shorting pins and slots along the double-sided parallel strip line (DSPSL). The periodically loaded shorting pins act as the VP radiator elements of the antenna, and also have the function to make the normalized phase constant of the original DSPSL equal to one so as to meet the endfire radiation condition. A series of slots etched on strip conductor around shorting pins are utilized to improve the radiation efficiency of the antenna in addition to its vertical polarization. In this way, the VP endfire LWA can be eventually realized with efficient radiation on a low-profile substrate. In our design, the mechanism of the proposed antenna is first analyzed based on the equivalent circuit model to investigate the effects of pins and slots on the antenna operation. Then, the attenuation properties of the antenna are detailly discussed under various structural dimensions to explore the relation between its physical sizes and radiation capability. Finally, a prototype is designed and fabricated to verify the predicted properties. The measured results match well with the simulated ones, thus demonstrating and confirming that the proposed endfire LWA can indeed realize efficient radiation with vertical polarization based on a single low-profile substrate.