Jonathan, who lives in Alwinton, was invited to the event in recognition of the 13 years he has served in the TA and how his employer Ward Hadaway supports his work in the Territorials.
The 36-year-old is an Officer in D (Northumberland Hussars) Squadron The Queen's Own Yeomanry, which is based at Fenham Barracks in Newcastle.
On exercises, Jonathan is a Troop Leader with four Combat Vehicle Reconnaissance (Tracked) Vehicles, while he is also involved in recruiting drives for the TA.
Jonathan and his wife Jane and Ward Hadaway's head of countryside Christopher Hewitt and his wife Annette were invited to the garden party, attended by the Princess Royal and Prince Edward.
Jonathan said: "It was a fantastic day. The weather was perfect, the Buckingham Palace gardens were beautiful – and huge – and the whole experience made me feel proud to be in the Territorial Army and to be recognised for the work we do.
"As well as performing a useful service for the country, being in the TA is useful for my role at Ward Hadaway in that it helps to reinforce leadership, planning and public speaking skills.
"I am very grateful to Ward Hadaway for allowing me to continue serving with the TA whilst working for the firm and this was a great opportunity to thank them for the support that I receive."