He began his career in the oil industry in 1980, working both as a journalist and as oil broker in London and New York, with Petroleum Argus, Eperon Petroleum, Albion Oil, Petroleum Intelligence Weekly and OPEC Listener, before joining MRC Business Information Group in Oxford. There, in 1992, he created BunkerNews.
In May 2003, Llewellyn founded Petrospot Limited, a dynamic independent publishing, training and events organisation focused on the maritime, energy and transportation industries. Petrospot publishes Bunkerspot and a growing library of books and ebooks. It also organises training courses, seminars and conferences worldwide, including the renowned Oxford Bunker Course and Maritime Week Americas.