The modern mince pie is quite a lot different from the traditional one, and originally they did have meat inside. They probably began to be eaten in the Middle Ages; with increased trade from the Middle East, and even with the wars that were fought there during the Crusades, from about the 12th century spices of all kinds began to be available in England and other European countries. This was very important because spices often have a preservative effect, which meant that meat dishes could be kept longer, as well as offering a lot of new and delicious flavours.
The mince pies that began to appear in England in the Middle Ages would typically contain minced or finely chopped meat, with dried fruit - also often imported from foreign countries – and mixed spices which gave the pies a sweet taste in spite of the meat. You can find some sweetened meat-based pies in parts of the Middle East even today, which could give some idea of what an early mince pie may have tasted like. You can also find a 15th century recipe here – it includes minced pork or veal, dates, currants and a mix of spices including saffron and ginger, and the pie would be rectangular in shape rather than the little round ones we prefer today. So you could always have a go at making one – the ingredients are readily available today.
Even in the 17th century it was still expected that a mince pie would be savoury. Making them was a lot of work – there are references in the 17th century diary of Samuel Pepys to how tired his wife was after ‘seeing the maids’ making the mince pies, which apparently took most of the night. (The maids were probably a lot more tired than Mrs Pepys, but he doesn’t mention this!)
What caused the birth of the sweet, meat free mince pie was probably the horrific legacy of slavery. In the 18th century the growth of the huge sugar plantations in the West Indies, for example, which were almost all staffed through slave labour, meant that sugar was available, and increasingly affordable, in many parts of the world – and in England as in other countries, this led to an increasingly sweet tooth among the population. Sugar continued to be a luxury for a long time, but it was after sugar began to be available that new recipes for mince pies began to appear without meat, but relying heavily on sugar and sweetened fruit instead. By the mid 18th century recipes had begun to appear in which using meat in your mince pies was optional. This happened more and more as sugar was used more, even though in the late 18th and early 19th centuries some people began to avoid using sugar as a protest against slavery and the conditions on the sugar plantations.
As we move into the 19th century we find that mince pies are now being made without meat as a matter of course. By this time the recipes are quite similar to the ones of today; in particular, it had become usual to use suet in the mixture, which gives a rich flavour and also helps to preserve the mincemeat, and also to include brandy.
Today mince pies are as popular as ever. There are any number of different recipes, for instance you can get vegetarian versions using vegetable suet, and some cooks use different fruit flavours such as cranberry, but most people still opt for the classic Victorian style mince pie – Delia has a delicious modern take on this recipe, for example.