Water, methanol and mixtures (20–80%) thereof have been used for the extraction of phenolic compounds from Irish brown seaweed Himanthalia elongata for studying its antimicrobial and antioxidant properties. The 60% methanolic extract exhibited significantly (P < 0.05) the highest value of yield (6.8 ± 0.24%), total phenol (286.0 ± 4.61 mg gallic acid equivalents/g), flavonoid (109.8 ± 2.68 mg quercetin equivalents/g) and condensed tannin content (35.6 ± 1.03 mg catechin equivalents/g). Antimicrobial activity of 60% methanolic extract tested from disc diffusion and broth dilution methods was effective against various food spoilage and pathogenic bacteria studied. The same extract exhibited statistically highest reducing power and antioxidant capacity against DPPH radical, metal ions, lipid peroxides and hydrogen peroxide radicals. The UV-visible spectroscopy showed absorption maxima at 205 and 260 nm and the presence of hydroxyl group (3,431 cm−1) and an aromatic ring (around 1,465, 1,505 and 1,624 cm−1) in Fourier Transform Infrared spectroscopy, suggested the presence of phenolic compounds in the extract.