The plants that we use as food in our daily diet and as risk preventers against many diseases have many biological and pharmacological activities. The heat treatments applied during the cooking of the plants cause changes in the phytochemical content and bioactivity. In this study, the phytochemical fingerprint and biological activities of raw and heat-treated extracts of Ornithogalum umbellatum L., which is widely consumed in the Black Sea region, were investigated. The bulb and leaf parts of the plant consumed as food were dried in an oven at 35 °C and then ground into powder. For heat treatment, the plant was boiled at 100 °C for 20 min. Differences in phytochemical contents of raw and heat-treated extracts were determined by ICP-MS, LC-MS/MS, and FTIR analysis. Biological activity was investigated with antiradical, antimicrobial, antimutagenic and antiproliferative activity tests. In this way, the effect of heat treatment on both the phytochemical content and biological activity of the O. umbellatum extract was determined. Gallic acid, procateuic acid and caffeic acid were found as the main compounds in the O. umbellatum extract, while the presence of procateuic aldehyde, vanillin and kaempferol in minor proportions was determined. There was a significant decrease in phenolic compound levels after heat treatment and gallic acid content decreased by 92.6%, procateuic acid content by 90% and caffeic acid content by 84.8%. Significant differences were detected in macro and micro element levels after heat treatment in ICP-MS results. While Cd, Ba and Zn levels of the raw extract increased; Na, Mg, K, Fe, U, Co levels decreased significantly. In FTIR spectrum, shifts and disappearances were observed in some of the vibrations and the emergence of new vibrations was also determined after heat treatment. Raw extract exhibited strong scavenging activity against HO and DPPH and had a broad spectrum antimicrobial property. As a result of heat application, regressions were detected in antiradicalic, antibacterial and antifungal activities. Antimutagenic and antiproliferative activities were determined by the Allium test and a significant decrease in both activities and loss of activity against some chromosomal abnormalities were determined after heat treatment. While the antiproliferative activity of the raw extract was 20%, the activity of the heat-treated extract decreased to 7.6%. The raw extract showed the strongest antimutagenic effect with 69.8% against the unequal distribution of chromatin. Similarly, the antimutagenic activity of the extract, which reduced the bridges by 56.1%, decreased to 0.74% after heat treatment and almost lost its antimutagenic activity. The biological activities of raw O. umbellatum are closely related to the major compounds it contains, and the decrease in the levels of these compounds with the effect of heat was reflected in the activity. Studies investigating the phytochemical contents of plants are very important and the studies investigating biological activities related to phytochemical content are more remarkable. In this study, the phytochemical fingerprint of O. umbellatum was determined, its biological activities were related to the compounds it contained, and the biological activity was found to be heat sensitive.