The campaign is targeting supporters from the Dogs Trust database, mainly 35-plus women who have supported the charity in the past.
The mailer uses a Valentine-themed red and pink palette and coloured hearts. A bright red envelope adorned with hearts and marked "special delovery" contains a Valentine's Day card with a picture of Collie-cross George asking recipients to "Be my Valentine".
The card unfolds to show a letter from George revealing how after 10 years of being loved and looked after he found himself without a home when his owner died.
He goes on to explain how donations from supporters help the Dogs Trust give dogs like him a second chance at happiness and how his Valentine's wish is for "all lonely, abandoned dogs to find love".
The back of the card provides examples of how anything from £12 to £100 can pay for little things like food and treats to medical care and housing.
The pack also includes a reply card with a prepaid envelope to give supporters the opportunity to send George a Valentine's card along with their donation.
Phil White, Mather's business director, said: "The objective of the mailing is to illicit a response from our predominantly female audience.
"By using a Valentine's Day themed mailing, the reader can emotionally connect with the plight of dogs whose love has been lost."
Mather was set up in 2007 as a media-neutral agency within Ogilvy Group.