Pet food has been commercially produced since about the late 1800's. James Spratt an electrician from Ohio is credited with being in London (to sell lightning rods) and observing the dogs there being feed ship's biscuits.
A ship's biscuit or hard tack (a more modern slang name for ship biscuit is dog biscuit) is a cracker made of flour, water and salt. Baked hard to withstand long periods (years) of storage and harsh travel conditions. It was generally softened by being put in coffee, water or whiskey. Hard tack fed to the soldiers during the Civil War was leftover for the Spanish American war 12 years prior. Presumably if the sailors were feeding it to the dogs they had no need for it.
Until that time dogs had been fed table scraps and other foods from the human plate. Commercially sold meat for animals was found up to and through the early 1900's. People in the city bought mostly horse meat, not suitable for people from street vendors, to feed their dogs and sometimes cats. http://www.museumoflondonprints.com/image/68598/john-galt-cats-meat-man-c-1901 Thus dogs, and sometimes cats, had been fed for a long time.
New research suggests that dogs were first domesticated between 18,800 and 32,100 years ago in Europe. That same study finds that the modern dog has little to do with the modern wolf instead descending from another wolf like animal no longer in existence. Surprisingly, researchers did not find a population of modern wolves that might have given rise to modern dogs. In fact, modern dogs have more genetically in common with ancient wolves than the ones we see today. According to research done by Olaf Thalmann, researcher at Finland's University of Turku, who led the study, "It's quite likely that the population of wolves that gave rise to modern dogs does not exist anymore,". This may also help explain why dogs through their domestication have been breed to use both meats and starch in their diet. So where does that leave us for dog food……
So for about 1/156th of their existence dogs have been fed a processed food made exclusively for them. After Spratt, several formulas sprouted up until by about 1890 pet food was being manufactured in the United States. Most of these were biscuit type and the iconic Milk Bone began manufacturing in 1908. After World War I canned horse meat was added to dog lines. Cat food and canned food made up the majority of pet food sales up until World War II. During World War II metal (tin) was rationed and pet food was classified as "non-essential", canned pet food wasn't available. Production changes were made and by 1946, dry food represented 85% of the market. In the late 1950's the modern process of extrusion was used in manufacturing dog food to create a lighter, larger kibble and dog food as we know it now came into being. Well over forty national manufacturers of pet foods exist in the United States now with countless brands being offered.