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Mufolinin A (1), a ring A-secorearranged limonoid with an unprecedented ethyl at C-10 and novel 6/6/6/5 fused-ring skeleton, together with three new potential precursors (ring A-secolimonoids, 2–4) were isolated from Munronia unifoliolata. Their structures and absolute configurations were confirmed by nuclear magnetic resonance (NMR), high resolution electrospray ionization mass spectroscopy (HRESIMS), X-ray crystallography, electronic circular dichroism (ECD) calculations and NMR calculations with DP4+ analyses. The unprecedented ethyl group of 1was hypothesized to be derived from methyl migration and ring reduction rearrangement of ring A-secolimonoid 4. Compounds 2and 4showed significant multidrug resistance (MDR) reversal activities in MCF-7/DOX cells with reversal fold (RF) values of 13.1 and 8.0, respectively.