This study presents the development of a high-performance ultrathin heat pipe (UTHP) with a thickness of only 0.40 mm, featuring an innovative “copper groove–plant fiber” hierarchical composite wick. The structure utilizes chemically etched copper micro-grooves (depth: 0.16 mm) as a structural backbone, with degreased cotton fibers embedded via geometric confinement by the groove ridges—eliminating the need for high-temperature sintering or adhesives and thereby avoiding interfacial thermal resistance associated with conventional composite wicks. Using deionized water as the working fluid, systematic experiments were conducted to investigate the synergistic effects of etching parameters, filling ratio (20.3%, 30.6%, and 40.8%), and orientation angle (–90°, 0°, 90°, 180°) on thermal performance under heating powers ranging from 1 W to 10 W. The results show that groove depth is approximately linearly proportional to etching time and can be described by a quadratic polynomial function of etching temperature; increasing etching temperature while shortening duration effectively enhances surface roughness and wettability, providing a favorable foundation for capillary transport. Among the tested conditions, the UTHP with a 30.6% filling ratio exhibits the best overall thermal performance: it achieves the lowest total thermal resistance of 0.75 K/W at 7 W in horizontal orientation, corresponding to an equivalent thermal conductivity of 11,500 W/(m· K). Transient response tests under 0→7 W step heating further confirm that this filling ratio yields the most balanced temperature rise across symmetric evaporation-zone sensors, indicating uniform liquid distribution and stable phase-change behavior. Orientation tests reveal that thermal resistance is highly sensitive to gravity direction: total resistance is minimized at 90° (favorable gravity) and maximized at –90° (adverse gravity). Detailed resistance decomposition shows that condensation resistance dominates under adverse or inverted orientations (contributing over 50% of total resistance), whereas evaporative resistance becomes the primary limitation (~69%) in horizontal or favorable-gravity configurations. These findings demonstrate that the proposed UTHP successfully integrates structural compactness, simplified fabrication, and excellent thermal performance, offering a promising, low-cost, and eco-friendly solution for thermal management in ultra-thin electronic devices.