This study aimed to use a shotgun proteomics approach to investigate proteome changes between two finishing diets (pasture or concentrate) and two meat aging methods (wet- or dry-aging) in 60 striploins (Longissimus thoracis et lumborum) from British breed steers (n = 15 from pasture and n = 15 from concentrate). Bag dry-aging led to higher abundances of proteins involved in muscle structure, energy metabolism, and proteolysis. Meanwhile, bag wet-aging led to increased levels of heat shock proteins (HSPs) and proteins associated with oxidative stress, suggesting a response to environmental stressors during aging. Concentrate-finished beef exhibited greater levels of glycolytic enzymes, which may contribute to improved tenderness and color stability, indicating a shift toward anaerobic metabolism. Pasture-finished beef increased the abundance of oxidative metabolism proteins, aligning with the greater physical activity of grass-fed cattle. TPM1, PGK1, TNNT3, GAPDH, ATP2A3, and PYGM were the top putative protein biomarkers in concentrate and dry-aged beef, followed by the overabundance of MYH7B in concentrate and wet-aged beef, with RPL23AY and IPO5 being the most significant in the combination of pasture-finished beef and wet-aging. This research provided a comprehensive understanding of how beef aging method and feeding systems influence the beef proteomes from wet- and dry-aging. It further identified, using PLS-DA-based chemometrics analysis, putative proteins as key discriminating biomarkers that warrant further investigation for dry-aged beef quality monitoring and authenticity.