Mulberry (Morus alba) by-products represent underutilized feed resources with potential for ruminant nutrition. This study evaluated the rumen fermentation kinetics and rumen digestibility of dried mulberry pomace (MP) and leaf (ML) to determine optimal inclusion strategies in dairy cattle diets. Mulberry pomace (MP) and mulberry leaf (ML) were sun-dried and incorporated at 50% substitution levels into total mixed rations (TMR) with varying concentrations (30%, 35%, 40%, 45%, and 50%) of neutral detergent fiber (NDF). This created ten treatment groups: 30NP through 50NP (pomace-supplemented, where the number represents basal TMR NDF%) and 30NL through 50NL (leaf-supplemented), plus control groups containing only MP or ML and five basal TMR controls (30N through 50N). Rumen fluid was collected from two non-lactating Holstein cows fitted with ruminal cannulas. Chemical analysis revealed that ML contained 19% crude protein and 27.4% NDF, while MP contained 14.9% crude protein and 35.8% NDF. The highest gas production was observed in the 45NP (43.20 mL) and 50NL (43.50 mL) groups. Results demonstrated that MP achieved optimal fermentation when combined with 40–45% NDF TMR (maximum total volatile fatty acid (VFA): 88.86 mmol/L in 40NP at 48 h), whereas ML performed best with 45% NDF TMR (45NL: 88.03 mmol/L total VFA), indicating these as the most promising treatment combinations for ruminant feeding systems pending in vivo validation. Acetate proportions were higher in ML groups (84–96%), while propionate ratios were elevated in MP groups. Both materials maintained optimal ruminal pH (6.2–6.8). In vitro NDF digestibility was significantly higher for ML, with differences increasing from 2.97% at 2 h to 16.44% at 240 h. In situ degradation of MP was nearly complete at 48 h, while ML reached maximum degradation at 24 h. These findings indicate the potential of MP and ML as valuable alternative feed sources for ruminants, particularly in TMRs containing 40–45% NDF.