A compact multiband monopole antenna is proposed for vehicular roof top shark-fin applications. The proposed multiband antenna covers 5G sub-6 GHz and LTE bands starting at 617 MHz to 5000 MHz and the higher GNSS band from 1559 to 1606 MHz as well as the V2X band at 5900 MHz. The presented antenna is a three-dimensional monopole antenna with two branches to cover the required bands with compact size to fit inside a roof top shark-fin. The long antenna branch covers the lower cellular frequency band from 617 to 960 MHz, while the short branch covers the higher frequency band from 1559 to 6000 MHz. The presented antenna is mounted on a double-sided FR4 PCB and is feeded through a short cable. The proposed antenna covers multiple frequency bands with compact size (H  x  L  x  W) of 58 × 37 × 17 mm3. The antenna is simulated and optimized, and then, a prototype is fabricated, and its radiation characteristics are measured when mounted on one-meter ground plane and on a vehicle’s roof. The maximum measured linear average gain is 3 dBi at 1900 MHz, and the maximum measured efficiency is 88% at 787 MHz. The active GNSS antenna gain is measured using an LNA with good isolation. A good agreement is achieved between the simulated and measured results when compared in terms of voltage standing wave ratio (VSWR), radiation patterns, linear average gain (LAG), and antenna efficiency.