William Matthias Longden was the only son of William Robert Longden and his wife Mary Ann, born Hall, who came from the Eastern Cape. His father died when he was only a year old. As a young adult he met the British imperialist Cecil John Rhodes in 1891 and joined the British South Africa Companu (BSAC) as Rhodes's agent at Mahlagazi, Zimbabwe. He was recalled in the middle of 1892 and after visiting his mother in Port Elizabeth undertook various other missions for Rhodes and the BSAC until 1895. At that time he was magistrate at Tuli. He was then sent to take charge of the new settlement of Melsetter (now Chimanimani), near the eastern border of Southern Rhodesia (now Zimbabwe). He became civil commissioner and magistrate there until at least 1911, but appears to have left the territory before 1916. His wife, Mary Doon Longden, died in 1942.
From the beginning of 1901 to the end of 1906 Longden maintained a second order meteorological station at Melsetter and sent his observations to the Meteorological Commission of the Cape of Good Hope. A summary of his results was published annually in the commission's Report. When he began his observations in 1901 there were only three other meteorological stations in the territory reporting to the Meteorological Commission. These were manned by Reverend C.O. Helm* (Hope Fountain, 1899-), E. Sharp* (Salisbury, 1899-), and Reverand van der Niet (Bulawayo, 1901-).