Mr Mariner says Dorset Council's plans to move services to a hub at Sturminster Newton is "not an acceptable alternative".
"It's a 20 to 25-minute drive on a good day. It would make it very, very difficult, if not impossible, because we're talking about an hours' round trip to drop her off," he says.
"It's not just the day care, the company, there's meals on wheels happening, there's other groups that meet here, it's a warm space in the winter, people come in, meet their friends, have a hot meal in the canteen, it's all very important really.
"It affects our lives greatly. If it were to close, it threatens my ability to work.
"I don't know what we would do without this day centre. It's a vital facility, it's a vital building."
Lorraine Hawley worked as a carer in Blandford for more than 20 years and says she is still a "friend" of the centre.
"Many people use it for all different services," she says.
"This is a vital day centre. If we didn't have it, then people would be isolated, they would lose friendships, they would lose support. The effects on the families as well, they would lose what they need to do in the day as well.
"We have disabled facilities, an enclosed garden for people with dementia, an outside garden for anybody who wants to use it.
"It's just a vital vital service and we need to keep it".
Dorset Council launched a three-month consultation in March on changing the services it delivers.
It said: "We've been encouraged by the robust debate and interest in our consultation process.
"At Blandford, we explored with people how connected the centre was to the communities in Blandford, and the way in which the centre supported a wider set of community connections.
"The engagement and community spirit was very much evident. As a result, we are now re-evaluating our current proposals for the site.
"Service users, their families, and carers, play a crucial role in helping us identify the best ways to support people who use the Blandford Day Centre or have additional needs within the community.
"We strongly encourage users to participate in the consultation process – it really is helping us to develop, change and improve our ideas for the future of the services and centres."
It said no final decision had been reached on service provision – but that "the conversations we had in Blandford will mean it is highly likely to be a different proposal that comes forward".
The petition launched by Mr Mariner gathered more than 750 signatures in less than 24 hours and been supported by several local councillors and Dorset North Conservative MP, Simon Hoare.
The Dorset Council consultation closes on 9 June.
