Marine seaweeds have increased their interest among researchers worldwide due to their beneficial health-promoting compounds such as phenolic compounds. Drying is one of the ancient methodologies to preserve food for a long shelf-life. It will help remove the moisture content from the seaweed and thus increase the bioactive compounds' shelf-life. This study was conducted to compare the fresh and dried seaweed's phenolic compounds of five different brown seaweeds, the seaweed were dried by using three different drying methods (freeze, oven, vacuum), and to evaluate their potential antioxidant activities. Subsequently, individual compounds were characterized and identified using LC-ESI-QTOF-MS/MS. Vacuum-dried Cytosphora sp., had high TPC (39.53 GAE mg/g), TFC (3.40 QE mg/g), DMBA (2.86 PGE mg/g), PBA (27.90 PGE mg/g), and FDA (0.85 PGE mg/g) whereas freeze-dried Cytosphora sp., had high radical scavenging activity in DPPH (47.66 TE mg/g). However, vacuum-dried Cytosphora sp., exhibited high antioxidant potential in FRAP (91.28 TE mg/g), ABTS (80.86 AAE mg/g), RPA (96.90 AAE mg/g), and ·OH-RSA (252.22 TE mg/g). Freeze-dried Ecklonia radiata had high antioxidant potential in FICA (14.70 EDTA mg/g) and freeze-dried Sargassum sp., exhibited high total antioxidant activity (29.29 AAE mg/g). The total phenolic and total flavonoid content were highly correlated with antioxidant potential. Construction of the Venn diagram demonstrated high phenolic profiling of unique compounds in Cystophora sp., and in the vacuum-dried seaweeds followed by the freeze-dried seaweeds. The LC-ESI-QTOF-MS/MS identified and characterized 22 compounds in vacuum-dried and 20 compounds in oven-dried seaweed species. While 18 and 5 compounds were identified in freeze-dried and fresh seaweed samples, respectively. Our results will provide vital information to industries including food, pharmaceutical, and nutraceutical for upscale drying and extraction of phenolic compounds.