Designing minimally invasive, defect-free coatings based on conformal graphene layers to shield metals from both abiotic and biotic forms of corrosion is a persistent challenge. Single-layer graphene (SLG) grown on polycrystalline copper (PC-Cu) surfaces often have inherent defects, particularly at Cu grain boundaries, which weaken their barrier properties and worsen corrosion through grain-dependent mechanisms. Here, we report that an SLG grown via chemical vapor deposition (CVD) on Cu (111) single crystal serves as a high-performance coating to lower corrosion by nearly 4–6 times (lower than bare Cu (111)) in abiotic (sulfuric acid) and microbiologically influenced corrosion (MIC) environments. For example, the charge transfer resistance for SLG/Cu (111) (3.95 kΩ cm²) was 2.5-fold higher than for bare Cu (111) (1.71 kΩ cm²). Tafel analysis corroborated a reduced corrosion current (42 ± 3 µA cm⁻²) for SLG/Cu (111) compared to bare Cu (111) (115 ± 7 µA cm⁻²). These findings are consistent with the results based on biofilm measurements. The SLG/Cu (111) reduced biofilm formation by 3-fold compared to bare Cu (111), increasing corrosion resistance, and effectively mitigating pitting corrosion. The average depths of the pits (3.4 ± 0.6 µm) for SLG/Cu (111) were notably shallower than those of bare Cu (111) (6.5 ± 1.2 µm). Surface analysis of the corrosion products corroborated these findings, with copper sulfide identified as a major component across both surfaces. The absence of grain boundaries in Cu (111) resulted in high-quality SLG manifesting higher barrier properties compared to SLG on PC-Cu. Our findings show promise for using the presented strategy for developing durable graphene coatings against diverse forms of corrosion.