Trenchard served in the 4th Battalion, The Royal Green Jackets from 1972 to 1980, reaching the rank of Captain.

He was appointed Senior Adviser to Her Majesty's Government on Japanese Financial Services in 2018.

For Kleinwort Benson International, he was general manager of its Tokyo branch from 1985 to 1988, president from 1988 to 1995 and deputy chairman from 1995 to 1996.

For the European Business Community in Japan, he was chairman of the securities committee between 1993 and 1995 and vice-chairman of its council in 1995.

For the Royal Air Force Benevolent Fund, Trenchard was member of council and trustee from 1991 to 2003 and again from 2006 to 2013.