Far fewer people of the twenty-first century know who John Dalberg-Acton was than the legions who are familiar with a famous quote attributed to him, known as "Lord Acton's Dictum." In a letter he wrote to Bishop Mandell Creighten, dated 1887, Acton made his famous pronouncement (in part): "Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men."

Born John Emerich Edward Dalbert-Acton, he was the only son of Sir Ferdinand Dalberg-Acton, Seventh Baronet. He was an English historian known more as a man of action than of letters. Educated privately due to his Catholicism (Catholics were not allowed to enroll at university), the young Acton developed a deep love of historical research and its potential critical power. He learned several languages early and fluently, and began at an early age to collect an exhaustive historical library. Liberal in his political thinking, Acton traveled a great deal, spending a good deal of time in the large cities of Europe and the United States. He was in attendance with the British representative to the coronation of Alexander II of Russia. In 1859 he was raised to the peerage by Queen Victoria and became the first Baron Acton. He became the editor of the Roman Catholic newspaper, The Rambler, during that same year. However, due to his independent thought and liberalism, the Church opposed his efforts. He eventually ceased publishing the newspaper but continued to contribute articles to other publications.

In 1865 he married the Countess Marie, daughter of the Bavarian Count Arco-Valley. The couple had one son and three daughters together. Acton was fascinated with American history, and was an avid supporter of the Confederacy during the American Civil War. Lord Acton became ill in 1901 and died the following June. He was succeeded in his title by his son, Richard Lyon-Dalberg-Acton, Second Baron Acton. His enormous library was purchased following his death by Andrew Carnegie, who presented it to John Morley, who gave it to the University of Cambridge.