In his introduction to the Penguin edition of Paradise Lost, the Milton scholar John Leonard notes, "John Milton was nearly sixty when he published Paradise Lost in 1667. [The writer] John Aubrey (1626–97) tells us that the poem was begun in about 1658 and finished in about 1663. But parts were almost certainly written earlier, and its roots lie in Milton's earliest youth." Leonard speculates that the English Civil War interrupted Milton's earliest attempts to start his "epic [poem] that would encompass all space and time."
Leonard also notes that Milton "did not at first plan to write a biblical epic." Since epics were typically written about heroic kings and queens (and with pagan gods), Milton originally envisioned his epic to be based on a legendary Saxon or British king like the legend of King Arthur.
Having gone totally blind in 1652, Milton wrote Paradise Lost entirely through dictation with the help of amanuenses and friends. He also wrote the epic poem while he was often ill, suffering from gout, and despite the fact that he was suffering emotionally after the early death of his second wife, Katherine Woodcock, in 1658, and the death of their infant daughter (though Milton remarried soon after in 1663).