Two new prenylated acylphloroglucinols, paleacenins A (1) and B (2), were isolated from the rhizome n-hexane and chloroform extracts of the fern Elaphoglossum paleaceum. Both compounds were found to possess the same geranylated filicinic acid moiety but have a different phloroglucinol ring substituent. Their structures were determined using 1H and 13C NMR spectroscopic, HRMS, and ECD analysis. The plant extracts and purified compounds were assayed for inhibition of monoamine oxidase (MAO) activity, and the n-hexane and chloroform extracts displayed 25.0% and 26.5% inhibition of MAO-A, respectively, as well as 42.5% and 23.7% inhibition of MAO-B, respectively. Compounds 1 and 2 exhibited IC50 values of 31.0 (1.3) μM for MAO-A and 4.7 (4.4) μM for MAO-B. Paleacenin A (1) showed a higher selective index (SI) toward MAO-B (SIMAO-B/MAO-A 0.1), and paleacenin B (2) exhibited selectivity to MAO-A (SIMAO-B/MAO-A, 3.5). The extracts showed cytotoxicity against a panel of prostate, cervix, breast, and colon cancer cell lines (IC50 values between 1.7 and 10.6 μg/mL); the pure compounds were more active against the prostate, cervix, and colon cancer cell lines. Paleacenins A (1) and B (2), with IC50 values of 46 and 41 μM, respectively, inhibited nitric oxide production by the RAW264.7 murine macrophage model.