Arley Arboretum was opened to the public in 2002 by Lord Lichfield after the last heir to the site, Roger Turner, put the condition in his will.
Lord Lichfield planted a tilia –Tree No 257 in the arboretum – to mark the occasion.
Staff, stakeholders, and visitors gathered at the estate to celebrate the anniversary on Friday, with an afternoon tea and the burying of a time capsule dedicated to the site.
They also celebrated the work of gardener Simon Thomas, who has worked at the estate for over 30 years.
Arboretum manager Helena Chisholm, from Kidderminster, said: "We're sharing centuries of heritage to the local community.
"Arley has a rich community culture with the arboretum being formed over 200 years ago, but was only opened to the public for the first time in its history by the RD Turner charitable trust in 2002.
"You can come here and enjoy the beautiful gardens and feel the essence of Arley."
Children from Upper Arley Church of England Primary School did presentations throughout the day and presented a time capsule devoted to the arboretum.
"It's trying to capture what's happening on the estate now," Helena added.
In the capsule are a list of staff names, a current map of the estate, pictures of the team, and newspaper articles from when Lord Lichfield opened the estate in 2002.
Annual Pass Holders will also be able to enjoy a free hot drink in the arboretum's tearoom between Wednesday and Friday.
All of this celebrates the legacy of the 30-acre site, which is home to over 300 species of tree - some of which are very rare, for example, the Whitty Pear.
The arboretum also has a walled Italian garden, a magnolia garden, and a 65-metre laburnum arch.