The Magna Carta, signed in 1215, first influenced British law by establishing the principle that everyone, including the king, was subject to the law. It laid the groundwork for due process and the rights of individuals against arbitrary power. The document served as a cornerstone for constitutional law and inspired later legal charters, helping to shape the English legal system and civil liberties. Its legacy continues to resonate in modern democracy and human rights, making it a pivotal moment in the evolution of law in Britain and beyond.