This work discusses the design and performance analysis of a small, multi-band slot-loaded octagonal microstrip patch antenna with a defected ground structure (DGS). The proposed antenna has been implemented using an FR4 substrate having a dielectric constant of 4.4, a thickness of 1.6 mm, and dimensions of 30 x 35 x 1.6 mm³. The evolution of this antenna begins with a basic circular patch radiating element with a radius of 7.2 mm, a resonant frequency of 9.69 GHz, a return loss of -15.44 dB, an impedance bandwidth of 1.19 GHz, and a gain of 4.5 dBi. To improve the antenna's operating parameters, the circular radiator has been modified to an octagonal shape and further incorporated with circular, rectangular, and inclined rectangular slots. This will introduce new resonant frequencies and increase the antenna's radiation efficiency. Extensive parametric studies have been conducted to examine the effects of slot shapes, slot orientations, and ground-plane changes. The designed antenna operates as a dual-band system with resonance at 6.5 GHz and 10.67 GHz. The antenna's return loss is - 24.28 dB, and the impedance bandwidth is 2.03 GHz at 6.5 GHz; at 10.67 GHz, the return loss and bandwidth are -13.03 dB and 770 MHz, respectively. The corresponding gains for both frequencies are 4.0 dBi and 4.2 dBi, respectively. The design offers substantial bandwidth improvements, compactness, and robust radiation properties, making it ideal for radar, satellite communications, microwave sensing, and related applications.