Lewis Alfred Munn was the eldest son of Captain A.H. Munn and received his education at the Thetford grammar school in Norfolk. He went to sea in 1867, became a master mariner in 1881, and continued his maritime life until 1894. For the last 20 years he was an officer, and later commander, in the Union Steamship Company of Southampton (later part of the Union-Castle Line).
In July 1895 Munn was appointed in the civil service of the Cape Colony as shipping master of East London harbour, and as official visitor to the lighthouses at Hood Point (to 1905) and East London Beacons (to 1899). He was employed as shipping master in the civil service only to 1901, but retained his post (as harbour master, or port captain) until his retirement in 1911. From the beginning of 1897 until the end of 1908 he was a regular meteorological observer for the Meteorological Commission of the Cape of Good Hope, in charge of the second order meteorological station East London West. A summary of his observations were published annually in the Commission's Report. During the Anglo-Boer War (1899-1902) Munn rendered valuable service in military transport operations at East London, for which he received the thanks of the Lords Commissioners of the British Admiralty. By 1908 he was a widower with four daughters. As a Freemason he was a member of the Buffalo Lodge.