Walter Synnot senior joined the 66th Regiment of Foot (Berkshire) as an ensign and was promoted to captain in 1797. By 1820 he was a half-pay captain in the 89th Regiment. In that year he led a party of 25 settlers from Wicklow, Ireland, to the Cape of Good Hope in the ship Fanny. He was accompanied by his second wife, Elizabeth, and his five children. The party was sent by ship to Saldanha Bay, to be settled along the Jan Dissels River near Clanwilliam. However, the site proved unsuitable as a result of poor grazing, insufficient water and a lack of suitable agricultural soil. Most of the settlers were consequently transferred to the Eastern Cape at the government's expense, but Synnot and some others remained. He was appointed deputy landdrost at Clanwilliam in 1821 and held this post to 1825. During these years he sent many bulbs and some plants of the family Geraniaceae to England. According to the London nurseryman Robert Sweet he sent more new and rare Cape bulbs to England than any other individual. Sweet named the genus Synnotia (Family Iridaceae) after him.
Synnot returned to Ireland in 1825 with a large collection of bulbs and seeds which he sold in England. In 1836 he emigrated to Tasmania, where he remained for the rest of his life.
Captain Walter Synnot should not be confused with his son, Walter Synnot junior, who was born in about 1808 and died in the Cape Colony in 1861.