The £1.5 million premises at Union Park is the hub for the first aid charity's activities in the West Midlands.
The event was attended by dozens of St John volunteers and staff, plus local dignitaries including John Crabtree OBE, Lord Lieutenant of the West Midlands, the High Sheriff of the West Midlands Michael Kuo, and the Mayor of Sandwell Councillor Ann Jaron.
The Princess Royal unveiled a commemorative plaque to mark the occasion and, in her capacity as St John's Commandant-in-Chief (Youth) she presented awards to four young St John volunteers.
Olivia Flood and Owen Bates were recognised as Super Badgers, while Grand Prior Awards went to cadets Rebecca Shepard and Reece Atkinson-Tulley.
Keith Hunter, St John's County Priory Group chairman, said: "We were delighted to welcome Her Royal Highness to take part in this exciting chapter in the ongoing development of our charity in the West Midlands.
"This is a multi-purpose, multi-use building – a tangible example of St John providing ‘state of the art’ hubs across the country, bringing together the many aspects of our work in well-used centres.
"We made this event into a celebration of youth in St John Ambulance and the involvement of our local units of volunteers who meet at West Bromwich ensured it was a very special day. Our youngest volunteers – Badgers aged between seven and 10 – sang the Princess a song they had been practising for weeks, which was a particular highlight. A lovely moment."
St John's West Bromwich buildings provide a home for adult and youth groups of volunteers. There is a garaging for 14 ambulances, along with additional support vehicles, plus offices that house regional events and logistics staff, as well as the national fleet and HR teams.
The property is also a major workplace and community training centre which has doubled capacity – the new facilities allow up to 24 people at a time to be trained in two classrooms, where the charity's previous venue in Roebuck Lane held just 12. The expansion will allow St John to teach more than 1,000 local people lifesaving skills each year.