On 15 October 1899, following the declaration of the Anglo-Boer War, Downie volunteered to join the Francistown Defence Force under the leadership of Umfreville Percy Swinburne.

Subsequently, he joined the firm of Haddon, Cotton & Butt, a Rhodesian shipping and forwarding house, where he later rose to be managing director.

John Downie later sold his interests in the firm, and in 1920 he became manager of the Salisbury Farmer's Co-operative, holding this post till the period of the end of Chartered Company Rule in 1923.

Following the replacement of Coghlan by Howard Unwin Moffat as premier, on 14 October 1927, Downie was reshuffled into the portfolio of Mines and Public Works, but continued to take a considerable interest in agricultural affairs.

On the other hand, he was able to reach a satisfactory arrangement with the Chartered Company to solve the problems of simplifying the collection of mining revenue, and he took an important part in settling some of the differences between miners and farmers.