I disagree with WYSIWYG. Sure, the system is quite complex, but he himself points out counter examples. I don't think it matters if the chloroplasts are maintained indefinitely (in Elysia they aren't and several peer-reviewed publications still call them photosynthetic), and I don't think it's that difficult to even put in all the genetic circuitry to maintain them if you really wanted to.
If you had asked whether a metazoan can be made photosynthetic, even if I hadn't heard of Elysia, I would say yes. But since you specify a mouse, the answer is no. Even if you had a green mouse with chloroplasts in its cells, it would gain very little nourishment from photosynthesis.
Cells cannot photosynthesize if they get no light. So, presumably, you would have thought to at least shave your mouse, because obviously the fur would block most light.
Even so, you must get rid of all the melanin pigment in the cell, because that will absorb light and waste its energy. But even if you used albino mice, the several layers of dead skin cells will still prevent light from reaching the living, photosynthetic cells. What's worse is that by introducing chloroplasts, you have counteracted the solution to the melanin problem - now all the dead cells are also full of very opaque green pigment, so 99% of the light is being absorbed by chloroplasts in dead cells.
The only way to solve this problem is to constantly scrub the skin of the mouse raw. This would be very painful and dangerous for the mouse, but let's assume you somehow managed to do it. You're still stuck. Even if the skin cells produce a lot of glucose, that glucose will not be effectively transported. Firstly, it will be the outermost cells that have the most concentrated glucose, but the inner cells are the ones in close proximity of blood vessels. Even if you somehow solved the transport issue, the veins coming from the skin do not go into the liver, they go to the heart. You will now end up with diabetes-like problems in many organs because of absurdly high blood glucose concentrations. Let's say you even solved this problem by doing surgery to dramatically rework the "plumbing" of your mouse, and adding glucose transporters to skin cells. Now you have to deal with the most fundamental problem of them all: It's probably not a coincidence that photosynthesis did not evolve in animals.
Photosynthesis works for plants because they have much lower energy needs. Even so, they must go to great lengths to maximize their surface area with many leaves. A mouse has evolved to minimize its surface area, because mice would much rather not waste calories on heating up the atmosphere. But even if you yet further modified your mouse by somehow "adding" huge blankets of skin that must be spread out under a sunlamp, I still doubt that unless we are talking about several square yards of skin for each mouse, that the energy needs of a mouse can be serviced adequately by photosynthesis. As a very rough estimate, imagine you wanted a field of grass big enough that it grows back faster than a mouse can eat it (assuming the mouse can subsist entirely on eating this "grass"). How much area would it have? Certainly much more than the surface area of a mouse.
If I had to imagine a mammal that relies on photosynthesis, it would probably have a specialized organ just for this. For instance, it could be something like a bat, with thin membranous "wings" of great area, used not for flying but for collecting light. Or it could have a membranous "sail", attached to perhaps a bony "mast" - the mast can be raised by muscles to stretch out the sail, and the sail can be folded and stuffed into a body cavity which the mast then plugs when the sail is not in use.
Either way, this animal would have a very large membrane, rich in photosynthetic cells and blood vessels. It would be a very leisurely animal, like a sloth, using little energy, and spend most of the day basking in the sun with the sail extended, making sugar. At night, it would perhaps scavenge for dead insects (to get nitrogen) or somehow trap insects with little effort.
This animal would probably live very close to the equator, somewhere very warm. Perhaps it is a desert animal (which would explain the need for such a convoluted means of sustenance) or some sort of sea creature which surfaces during the day.
The fragile sail (so it would need to have a way of rapidly stopping blood flow) would have veins going to the liver, and there would have to be some very involved immune system adaptations to deal with the pathogen burden of having such an exposed tissue.
Needless to say, I cannot possibly imagine any lineage of mammal that such a bizarre animal could have diverged from; but if god set out to create a photosynthetic animal that survives in the wild, it would probably look somewhat like this (and absolutely not like a mouse).