From 1929 to 39, he worked in different Chinese cities including Yangtze ports, Hong Kong, Tientsin and Shanghai.

He was also chairman of associated companies such as Taikoo Dockyard and Engineering Co. Ltd., Taikoo, Sugar Refining Co. Ltd., Duro Paint Manufacturing Co. Ltd., Swire and MacLaine, Ltd., Cathay Pacific Airways Ltd. and the Hong Kong Aircraft Engineering Co. Ltd.

[1] Knowles was manager of Butterfield and Swire until he served briefly in the office of vice-chancellor of the University of Hong Kong, in succession to Sir Lindsay Ride, from 1964 to 1965.

[1] In recognition of his services to the colony and to the university he was awarded the honorary degree of Doctor of Laws in 1964.

[3] He held the position of executive director of Lloyd's Register of Shipping when he died suddenly on a business trip in Jakarta, Indonesia on 13 January 1969.