The dog dies.
After Marley knocks over the baby and is kicked out the house by Jennifer, she decides to take him back in and work around the problems together. John accepts a new job in Philly, with a luxury house they can now afford and lots of land for the dog and kids - and winters with lots of snow.
Now getting on in years, one day they find him alone outside the house. He is rushed to the vets and surgery is needed to untwist his stomach. The family realise how important he is to them, and they take a little more care of him now.
Unfortunately, not long after, he suffers the same condition. John has no choice but to allow the vet to put him down, but understandably he is affected badly by it.
Marley is burried near a tree in the front garden, and the family pay their respects - leaving behind pictures and mementos near the grave.