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.
1. Royal garden party
Queen Camilla pictured during the garden party at Hillsborough Castle. Photo: Kelvin Boyes /Press Eye
2. Royal garden party
King Charles meets Fr Gary Donegan during the Hillsborough Castle garden party. Photo: Kelvin Boyes /Press Eye
3. Royal garden party
Pictured at the royal garden party is Mayor of Mid and East Antrim Borough Council, Alderman Noel Williams. Photo: Kelvin Boyes /Press Eye
4. Royal garden party
King Charles and Queen Camilla receive a warm welcome to Hillsborough Castle. Photo: Kelvin Boyes /Press Eye