Christopher Reuel Tolkien is the third child and youngest son of J.R.R. Tolkien and Edith Tolkien.
He was named after his father's friend, Christopher Wiseman. He also sometimes uses his confirmation name, "John" as seen on his initials of maps of The Lord of the Rings, "CJRT". He was born in Leeds and raised in Oxford and went to the the Dragon School in Oxford and Oratory School in Caversham, Berkshire. Due to a heart ailment he was forced to stay at home and work with a private tutor. He enjoyed watching stars with a telescope as well as a passion for railways.
In July of 1943 he entered the Royal Air Force and in 1944 he went to South Africa to train as a pilot. His absence did not however slow his contributions to his father's works as his father continually sent him parts of The Lord of the Rings to go over. In 1945 he returned to England and was stationed in Shropshire and later that year he returned to Oxford. On October 9th, 1945 his father informed him that the Inklings wished to consider him a permanent member. The task of reading The Lord of the Rings to the Inklings was passed on to Christopher and it was generally agreed that he was a better reader than his father.
In 1946 Christopher returned to Trinity College to resume his studies and reading English. For a while his tutor was none other than C.S. Lewis. His thesis was a translation of The Saga of King Heidrek the Wise and he received his B.A. in 1949. Christopher also became a lecturer in Old and Middle English as well as Old Icelandic at Oxford. He worked as an editor on Chaucer's Canterbury Tales, the Pardoner's Tale, and the Nun's Priest's Tale. From 1963 to 1975 he was a Fellow of New College, Oxford but resigned when he began to devote his time to his father's literary affairs.
After his father's death, Christopher embarked on organizing the masses of his father's notes, some of them written on odd scraps of paper a half-century earlier. Much of the material was handwritten, frequently a fair draft was written over a half-erased first draft, and names of characters routinely changed between the beginning and end of the same draft. Deciphering this was an arduous task, and perhaps only someone with personal experience of J. R. R. and the evolution of his stories could have made any sense of it; even so, Christopher has admitted to having to occasionally guess at what his father intended.

With the help of Guy Gavriel Kay he managed to compile The Silmarillion in only four years. During this time he also edited is father's translations of Sir Gawain and the Green Knight, and Sir Orfeo. He also worked on the Nomenclature of The Lord of the Rings which was first published in 1975 as Guide to the Names in The Lord of the Rings in A Tolkien Compass.

Christopher spent the years after continuing to study his father's works and taking the responsibilities of the Tolkien Estate. He recorded portions of The Silmarillion in 1977 and 1978 which was issued by Caedmon Records, New York. In 1979 he wrote about his father's illustrations and drawings for their publication in Tolkien calendars and Pictures by J.R.R. Tolkien. Through 1980 and 1983 Christopher edited Unfinished Tales, The Letters of J.R.R. Tolkien, The Monsters and the Critics and Other Essays, and The Book of Lost Tales Part 1 which was the first volume in his twelve volume series of The History of Middle-earth, the last of which was published in 1996. In 1998 he edited a new edition of Tree and Leaf including the poem Mythopoeia. His latest publication has been the editing of The Children of Húrin which was published in 2007.
Christopher's first wife, Faith (1928) took an English degree at Oxford and they had one son, Simon Tolkien. A bust of Tolkien by Faith was exhibited at the Royal Academy: Tolkien paid for its casting in bronze. It is now in the English Library in Oxford.

Christopher's second wife, Baillie (1941) is Canadian, and is the daughter of Winnipeg surgeon Alan Klass, and Helen Klass (née Jacob). She has a BA in English from the University of Manitoba and an MA from Oxford. She worked as JRR Tolkien's secretary and was responsible for the section on poetry in the 1965 index to The Lord of the Rings. She later edited The Father Christmas Letters. She and Christopher have two children, Adam Tolkien and Rachel Tolkien.