From 1984 until 2006, John was in full-time practice as leading counsel based in London. During that period, he also developed an international arbitration practice as an arbitrator, and has acted in that capacity in London, Paris, Geneva, Stockholm, Dubai, Hong Kong, Singapore and the Caribbean.

As well as appointments in ad hoc arbitrations, he has frequently been appointed either as chairman or as co-arbitrator in references under the International Chamber of Commerce Rules, under the UNCITRAL Rules and under the LCIA Rules.