The mechanical properties of materials depend strongly on crystal structure and defect configuration. Here we measure the strength of suspended single-crystal and bicrystal graphene membranes prepared by chemical vapour deposition. Membranes of interest are first characterized by transmission electron microscopy and subsequently tested using atomic force microscopy. Single-crystal membranes prepared by chemical vapour deposition show strengths comparable to previous results of single-crystal membranes prepared by mechanical exfoliation. Grain boundaries with large mismatch angles in polycrystalline specimens have higher strengths than their low angle counterparts. Remarkably, these large angle grain boundaries show strength comparable to that of single-crystal graphene. To investigate this enhanced strength, we employ aberration-corrected high-resolution transmission electron microscopy to explicitly map the atomic-scale strain fields in suspended graphene. The high strength is attributed to the presence of low atomic-scale strain in the carbon-carbon bonds at the boundary.