Mint sauce has a rich history in the UK and particularly in England. Traditionally served with lamb, it differs from the mint jelly that sometimes accompanies lamb dinners in North America. (Mint jelly is a popular alternative to mint sauce, but it's considered thicker and sweeter than sauce.)
Mint sauce itself dates back to medieval times, when "sweet and sour" sauces such as those made from mint were quite popular. Due to the historically high price of sugar, there was often little distinction between sweet and savory.
Other research suggests that "mint olive oil dipping sauce" dates back to ancient Greece. Whatever the case, the tradition spread throughout France and Italy, where mint was easily accessible.
The legend goes that, for England, mint sauce became a staple for lamb dinners thanks to Queen Elizabeth I. To stop her subjects eating lamb and mutton to help the wool industry, she declared that the meat could only be served with bitter herbs.
But cooks discovered that mint made the meat taste better, not worse, and so it quickly became a time-honored tradition to serve meats such as lamb with mint sauce.
A British favorite since the 16th century, some research even suggests that the association of lamb with mint sauce dates back to the Israelites, who ate the meal on the eve of their exodus from Egypt.
According to Washington State magazine, the reason for the pairing of lamb and mint sauce isn't merely seasonal or cultural, it's scientific. "Mint is rich with branched-chain ketones, chemically related to the branched-chain fatty acids released during cooking lamb. Foods that share similar compounds and chemical structures taste better together ... If lambs consume fresh clover and ryegrass, they’re going to pair even better with mint. This diet produces a particular compound that chemically bridges the gap between the lamb’s fatty acids and the mint’s ketones with an aroma that further complements the pairing."
Lamb with mint sauce is also a nutritious meal that is associated with spring, a popular Easter meal in the UK. "Herbaceous and refreshing, mint stands up to the richness of the naturally tender roast, which comes from sheep less than a year old and tastes unlike any other cooked meat. With its crisp browned surface and velvety, juicy interior, this dramatic-looking dish makes an elegant centerpiece for springtime suppers and celebrations. The bone-in leg of lamb’s presentation is particularly impressive. Savory, succulent, and exquisitely flavored, lamb is high in protein, B vitamins, zinc, and iron, and contains very little marbling. Fat congregates at the edges of cuts, making them easy to trim."