Among those in attendance was PSNI Detective Chief Inspector John Caldwell who made his first public appearance since a murder bid in Omagh earlier this year.
Mr Caldwell was seriously injured after he was shot several times at a sports centre in the Co Tyrone town in February after coaching a youth sports team.
The Queen spent some minutes speaking with Mr Caldwell during the garden party and he is understood to have had a private audience with the King earlier.
5. Royal garden party
Enjoying the garden party at Hillsborough Castle. Photo: Kelvin Boyes /Press Eye
6. Royal garden party
King Charles and Queen Camilla pictured at Hillsborough Castle. Photo: Kelvin Boyes /Press Eye
7. Royal garden party
King Charles meeting guests at the royal garden party. Photo: Kelvin Boyes /Press Eye
8. Royal garden party
Guests enjoy the moment they met the King at Hillsborough. Photo: Kelvin Boyes /Press Eye