The cold-pressed blackberry, broccoli, carrot, cucumber, and milk thistle seed flours were extracted with 100% ethanol and examined for their phytochemical compositions, total phenolic contents, ABTS•+ and relative DPPH• scavenging capacities, and anti-proliferative activities in HCT116 and SW480 colon cancer cells. Eleven, eight, ten, and thirteen compounds were tentatively identified in the blackberry, broccoli, carrot, and milk thistle seed flour extracts, with ellagic acid, glucoraphanin, kaempferol-3-O-rutinoside, and silychristin isomers being the primary components in each, respectively. Milk thistle seed flour extract had the greatest total phenolic content. Blackberry seed flour extract possessed the strongest free radical scavenging capacities against both DPPH• and ABTS•+. Milk thistle seed flour extract was the only extract capable of significantly suppressing the growth of SW480 colon cancer cells, but not HCT116 cells.