Porous membranes/mats with both high porosity and flexibility are strongly required for practical applications. Metal–organic frameworks (MOFs) are excellent candidates owing to their ultra-high porosity compared to most of the porous materials. However, the intrinsic fragility of MOF crystals makes the fabrication of flexible MOF-only membranes/mats rather challenging. This article presents a facile and general strategy to prepare flexible self-supported MOF-only mats that have exceptionally high porosity compared with the reported flexible membranes/mats (even remarkably higher than that of MOF powders), and can be produced on a large scale, easily manipulated, and highly adaptive for a variety of uses. A wide range of flexible MOF mats including MIL-53(Al), MIL-53(Al)-NH₂, HKUST-1, ZIF-8, ZIF-67, UiO-66 and MIL-88B(Fe) have been prepared by the complete phase transformation of electrospun fibrous metal oxide mats. The hierarchical porous structure endows the mats with extra large surface area, excellent pore accessibility and efficient mass transport. Water treatment, air filtration and catalytic applications have been demonstrated, revealing the significantly enhanced performance of the flexible self-supported MOF mats over supported ones and/or MOF powders. This work overcomes the longstanding challenge of processing pure MOFs into a flexible and tailorable form, opens a new opportunity for MOF-only materials in high-performance devices, and greatly broadens the scope of functional membranes/mats.