The son of a Cumberland mine owner, Miller was educated at Rugby School and Oxford, where he gained a double first, and was called to the bar in 1889, but never practised law. He married Ada Mary Wright in 1895, and in 1896 published The Balkans, followed in 1898 by Travels and Politics in the Near East.
Together the couple lived in the Ocean View Hotel in Durban, South Africa, for the rest of their lives. Miller died there in 1945, while Ada Mary surviving him by five years. They had no children.
Miller was particularly interested in the Frankish period of Greek history, covering the Crusader principalities established on Greek soil following the Fourth Crusade. He was among the most eminent scholars of the field in the early 20th century, and produced a number of "landmark" studies.
Although his work displays a "romantic view of the Crusades and the Frankish expansion into the Eastern Mediterranean" typical of 19th-century Western trends on the subject, and is considered "clearly outdated" given the research produced in recent decades, it has had a major influence and remains widely used to this day. Particularly the 1908 The Latins in the Levant has "remained for decades the standard English-language narrative account of the period", and is "still the main reference for undergraduates in search of information on medieval Greece". Its influence has also been felt in Greece, where already in 1909–1910 the Greek scholar Spyridon Lambros issued an expanded Greek translation of the work.
The Balkans: Roumania, Bulgaria, Servia, and Montenegro. New York and London: G. P. Putnam's Sons and T. Fisher Unwin. 1896. Retrieved 19 September 2018 – via Internet Archive.
Travels and Politics in the Near East. London: T. Fisher Unwin. 1898. Retrieved 19 September 2018 – via Internet Archive.
Greek Life in Town and Country. London: George Newnes, Limited. 1905. Retrieved 19 September 2018 – via Internet Archive.
The Latins in the Levant. London: John Murray. 1908. Retrieved 19 September 2018 – via Internet Archive.
The Ottoman Empire and its Successors, 1801-1922 (2nd ed.). Cambridge: At the University Press. 1923. Retrieved 19 September 2018 – via Internet Archive., (4th ed), 1936
Essays on the Latin Orient. Cambridge: At the University Press. 1921. Retrieved 19 September 2018 – via Internet Archive.
History of the Greek People (1821-1921); with an Introduction by G. P. Gooch. London: Methuen and Co. Ltd. 1922. Retrieved 19 September 2018 – via Internet Archive.
Miller, William (1923). "XV: Greece and Aegean under Frank and Venetian Domination (1204—1571)". In Bury, J.B. (ed.). Cambridge Medieval History. Vol. IV: Eastern Roman Empire, 717–1453. Cambridge: Cambridge University Press. pp. 432–477.
Tsougarakis, Nickiphoros (2014). "The Latins in Greece: A Brief Introduction". In Tsougarakis, Nickiphoros I.; Lock, Peter (eds.). A Companion to Latin Greece. Brill's Companions to European History. Leiden and New York: BRILL. pp. 1–22. ISBN 978-90-04-28410-4.