Mary Powell was 16 and John Milton 35 when they married in 1642. It seems a strange match -- not so much for the difference in age, which was not uncommon at the time, but because there are no readily discernible social or familial reasons for the marriage. Mary's father had been involved in a complex business transaction with Milton's father, and Milton continued to collect on the outstanding debt. Even stranger, the Powells were staunch Royalists, while Milton was an equally-devoted -- and much more important -- parliamentarian. Certainly the marriage surprised Edward Phillips, Milton's nephew and live-in student at the time, and our main source for this period of Milton's life.
It is an entirely reasonable assumption -- made by both David Masson and William Parker (the poet's definitive 19th and 20th century biographers, respectively), among others, that the marriage was one of sudden love rather than careful consideration.
However, the marriage turned out disastrously for Milton. Within two months of moving to London with her new husband Mary Milton returned to her parents' home. Repeated entreaties to return brought ever-more strident refusals. She would not return for nearly three years. The allegation of some kind of mental illness is entirely my own, but it certainly fits the facts -- and, as Parker points out, the experience of a teenage girl transported abruptly from a busy, familial country home to the spartan, urban accommodation of her bookish and decidedly bachelorish older husband, is likely to be destabilising.
During the time of Mary's absence Milton published four tracts (running to the hundreds of pages each) proposing and defending the idea of essentially no-fault divorce. Barbara Lewalski has used the divorce tractates to argue that Milton was not in love with Mary Powell: the arid marriage given as example in his pamphlets must, she assumes, describe his own. However, had Milton wanted a divorce, he did not have to jump through scriptural hoops. Desertion -- of which his was a clear example -- was grounds for divorce under Puritan law. Had Milton sought the end of his own marriage he could have had that in 1643.
However, Milton did not divorce Mary. When she came back in 1645 -- it is likely that political concerns, the Royalist cause by then being lost, prompted her family to resurrect their tie with a prominent parliamentarian -- Milton took her immediately back. They had four children in seven years, and Mary died three days after the birth of the last.
Scholars differ about whether Milton's Sonnet 23 refers to Mary or to Milton's second wife, Katherine Woodcock. Briefly, I do not find identification with Katherine convincing. First, “Child-bed taint” seems to apply much more easily to Mary than to Katherine: Mary died three days after the birth of her daughter; Katherine died of fever four months after the birth of hers. Secondly, the sonnet's line “and yet once more I trust to have full sight of her in Heaven” implies that Milton had seen the subject of the poem before he lost his sight. The earliest date for Milton's total blindness is February of 1652, shortly before Mary died, and the latest would be some time in 1654. A year or more, at a minimum, before he can be shown to have met Katherine. There are other, more technical arguments in favour of Mary, as well.
The allegation of distress following Anne's birth, and the consequent writing of Sonnet 23 then, several years before Mary's death, is entirely my own, for which I have no justification besides the dramatic.