Minor point: What you describe isn't usually called working memory. Working memory is what you can "keep in mind" at any one point. It lasts for a few seconds and then has to be refreshed, e.g. by repeatedly saying a phone number to yourself in your head. Working memory is more or less synonymous with short-term memory.

What you describe is long-term memory (everything beyond a few seconds is considered long-term).

Edit: Too slow. Some more justification: Very broadly, one hypothesis is that working/short term memory is stored in the currently present activity patterns of neurons, which fade/decorelate after a few secodns. Anything longer is thought to be stored in the weights of the synapses between neurons (there are alternative theories but I like this one).