The John Ramplin Charitable Trust was set up in memory of Mr Ramplin, who died in 2016.
After spending a career in banking he retired to Guernsey and became director of Guernsey Care Homes Holdings.
Mr Ramplin used to own Saumarez Park Manor care home, so often used to walk around the park.
So when Floral Guernsey chairman Pat Johnson approached the trust with the idea of the pergola, they agreed.
‘For this great man, a bench or a tree would be too understated, considering his generosity to the community,’ he said.
‘It was very substantial, what he gave to the community. He wanted to invest in the community and make Guernsey better.’
There used to be a pergola by the rose garden on the south side of the park in the early 20th century, but it had been removed by the Second World War.
Mr Johnson said they started planning a replacement in November.
It was delivered in June and then erected over the summer.
More than eight tons of green oak were used to create the custom-made structure.
The wood will need to be treated with teak oil once every six months for the next two years, which will ensure the wood will last.
The stone base of the old pergola had survived, but had been badly damaged by agapanthus roots over the years.
That has now been repaired.
There are no plans to grow plants over the wooden structure. Mr Johnson said it was decided it was better to treat the pergola like a piece of sculpture and leave it uncluttered, so the public could appreciate its shape.
He added that Floral Guernsey had received good feedback about the structure.
Every single comment we’ve had has been supportive,’ he said.
‘When you do something like this, you are not always sure how it will be received.
But this has been very positive.’