The structures of four leucinostatin analogues (<b>1</b>-<b>4</b>) from <i>Ophiocordyceps</i> spp. and <i>Purpureocillium</i> spp. were determined together with six known leucinostatins [leucinostatins B (<b>5</b>), A (<b>6</b>), B2 (<b>7</b>), A2 (<b>8</b>), F (<b>9</b>), and D (<b>10</b>)]. The structures of the metabolites were established using a combination of analytical methods including HRESIMS and MS/MS experiments, 1D and 2D NMR spectroscopy, chiral HPLC, and advanced Marfey's analysis of the acid hydrolysate, as well as additional empirical and chemical methods. Compounds <b>1</b>-<b>10</b> were evaluated for their biological effects on triple negative breast cancer (TNBC) cells. Leucinostatins <b>1</b>-<b>10</b> showed selective cytostatic activities in MDA-MB-453 and SUM185PE cells representing the luminal androgen receptor subtype of TNBC. This selective activity motivated further investigation into the mechanism of action of leucinostatin B (<b>5</b>). The results demonstrate that this peptidic fungal metabolite rapidly inhibits mTORC1 signaling in leucinostatin-sensitive TNBC cell lines, but not in leucinostatin-resistant cells. Leucinostatins have been shown to repress mitochondrial respiration through inhibition of the ATP synthase, and we demonstrated that both the mTORC1 signaling and LAR-selective activities of <b>5</b> were recapitulated by oligomycin. Thus, inhibition of the ATP synthase with either leucinostatin B or oligomycin is sufficient to selectively impede mTORC1 signaling and inhibit the growth of LAR-subtype cells.