"I'm of a certain age and have decided it's the right time," said Mary, who has led the archive team since 1997.
"I've been very privileged to work with some amazing documents we have in the collection and help secure them and make them more accessible. I have been very lucky in that I have been able to do a job that I really enjoy and completely believe in as a job.
"I think it's important that the history of Shropshire and its people should be preserved and that people should have access to that – the archives belong to everybody."
She said the most obvious change in her time has been the use of technology.
When she first came to Shropshire in 1990 from a job in Cardiff there were just two computers available and she would write things in pencil which would be given to a typist to type up on a typewriter.
"That seems so unreal now."
The online archives catalogue was launched in about 2000.
"That was the first time we had information on the internet that people could access. That has grown. Our customers who come through the door are really important, but our online customers across the world are people we need to provide services for as well.
"The other thing that has changed is the profile of history. Think about all the television programmes, indeed television channels, devoted to history.
"That used not to be the case. It was a much more specialised interest. The whole family history thing has taken off and is much more mainstream.
"We are in Shropshire lucky that so many early documents have survived. When I worked in South Wales there were very few medieval documents which survived. When I came to Shropshire there were boxes and boxes of them."
As for a favourite item in the collection, she says: "I don't know, but I've got a soft spot for a 12th century writ of King Stephen which we have. When you look at it it might have been written yesterday. The writing, the quality, and the penmanship is just amazing."
Currently archives staff are working from home but continuing to deal with inquiries. The Shropshire Archives building in Shrewsbury is shut and Mary says: "We don't know when we will reopen, but it will not be earlier than April 12."
Mary, who retires on March 26, was celebrating her retirement with a "virtual party" but is hoping that it might be possible to hold a get-together in the summer when the situation allows.
"I'd like to thank everybody who I've worked with. We have a very active Friends organisation. They have supported me all the way through which makes a big difference to what you can achieve.
"We are very lucky as the customers we deal with are fantastic and appreciate the service we provide, which makes it all worthwhile.
"I definitely want to continue to be involved in local history and have already agreed to be treasurer of a couple of organisations. We will see how it goes. I'm a bit wary of taking on too much."