An efficient gas chromatography–mass spectrometry (GC–MS)-based analytical method utilizing tert-butyldimethylsilyl (TBDMS) derivatives was developed and validated for the simultaneous quantification of 26 phenolic acids in plant extracts. The method demonstrated excellent linearity (r ≥ 0.997) within the range of 10–500 ng/mL, with good repeatability (%RSD ≤ 11.4%) and accuracy (–10.5% to 10.5% RE). Applying this method to 48 wild plant samples collected from island regions enabled the detection of 26 phenolic acids. Among them, caffeic acid, ferulic acid, and p-anisic acid were the most abundant and were particularly enriched in samples exhibiting potent muscle-enhancing activity in C2C12 myotubes. Multivariate statistical analysis, including PLS-DA and VIP scoring, identified key bioactive phenolic acids ferulic acid, caffeic acid, p-coumaric acid, and vanillic acid that were strongly associated with high bioactivity, whereas compounds like 2,6-dihydroxybenzoic acid and phloretic acid were more prevalent in low-activity samples. Additionally, seasonal and geographical factors were found to significantly influence phenolic acid profiles, with summer-collected samples and those from regions such as Palyeongsan and Ulleung-gun exhibiting higher concentrations of key phenolic acids. These findings collectively suggest that phenolic acids in island wild plants contribute to muscle function improvement and highlight their potential as functional dietary agents for sarcopenia management.