The proliferation and differentiation of myoblasts, as key regulatory processes for skeletal muscle growth, development, and in vivo balance, directly affect the economic value of livestock and poultry. Based on this, we examined the effects of Mettl21c (methyltransferase-like 21C) and carnosine (β-alanyl-L-histidine) on the proliferation and differentiation of ovine myoblasts. In this system, during the proliferation phase, Mettl21c overexpression reduced the EdU-positive cell rate and increased the apoptosis rate (p < 0.05), whereas interference had the opposite effect. These results indicate that Mettl21c inhibits myoblast proliferation and promotes apoptosis. At the transcript level, Mettl21c overexpression decreased Pax7 mRNA (progenitor marker) and increased Myf5, Myod1, and Myog mRNAs (MRFs, stage-specific myogenic markers). However, at the protein level, PAX7 increased while MYF5, MYOD1, and MYOG decreased. During the differentiation phase, interference with Mettl21c significantly reduced the cell fusion index and total myotube area (p < 0.05) while overexpression promoted these processes, indicating that Mettl21c promotes myoblast differentiation. At the same time, 1–2 mM carnosine was found to positively regulate myoblast proliferation, and carnosine increased the fusion index at 2 mM and total myotube area at 1–10 mM, which also positively regulated myoblast differentiation. However, Mettl21c perturbation altered the transcript levels of carnosine-metabolism genes (Carns1 and Cndp2), and carnosine treatment induced non-monotonic changes in Mettl21c mRNA expression. In summary, in primary ovine myoblasts, Mettl21c inhibits the proliferation of myoblasts and promotes their differentiation, while carnosine supplementation positively regulates both processes. Intracellular carnosine content and a functional interaction between Mettl21c and carnosine were not assessed in this study. These in vitro findings identify Mettl21c and carnosine as candidates for further studies of ovine myogenesis and muscle growth.