In reading about other matters entirely (fish in the Anglo-Saxon diet), I happened across a mention of Æthelflæd, a ruler. Wait, I said, that's a woman's name. And it appears that indeed she was. How did it come about that Mercia was ruled by a woman in the late 10th century, both before and after her husband Æthelred's death? This seems to be out of the ordinary for both the Anglo-Saxons and any of their neighbouring cultures.
She came to power through family relations, but more importantly by reputation and family reputation. You are right that in the 10th century female rulers were non-existant. Whats very interesting about her reign is that she was both West Saxon through her father, but also Mercian through her mother. Her being Mercian through her mother made it much easier to garner support among Mercian lords because while Mercia was essentially a vassal of Wessex, the lords still had pride and wanted to be lead by a Mercian.
Aethelflaed (sorry dont know how to the do that key). Was the daughter of the Alfred the Great. King of Wessex, the ruler who essentially broke Danish control of Britain after losing almost everything. Her Brother was Edward, who became King of Wessex after Alfred Died, and Liberated East Anglia, a danish held kingdom for decades before. She was Married off the AEthelred who was the Front runner for the king of Mercia (mostly because of Alfreds support), who in turn agreed to marry so that Wessex would help stabilize the kingdom and push the danes out. Alfred (and Edwards) dream was to unite the island under West Saxon Control.
Keep in mind that Mercia was heavily Occupied by Danes for much of the previous years and didnt really have a direct kingship line to follow... that is somewhat debatable, but at the very least there were certainly competing claims. The general consensus is that Aflred married her off to Ethelred because it gave him and his line access to the throne of Mercia.
There is some debate as the Ethelred's ability to rule. I have read a few sources that say he was a glutton and earting was all he cared about, and so left his wife to rule the kingdom. Others suggest he was a decent ruler in his own right but suffered from ongoing illness that made him unable to rule, and so left it to a strong willed wife. I find this claim to be far more believable. Either way, Aethelflaed became enormously popular among the Mercian lords and peasants alike for her strength in dealing with the danes, and her fairness as a civil leader. She is reported to have almost single handedly driven the danes out of Mercia. As mentioned above, she was also popular because she had Mercian heritage and this was a time when there were grumblings among Mercian nobles about the control Wessex was exerting on Mercia.
There is a ton of debate as whether she actually lead military campaigns or not, which may have also contributed to her reputation. Some suggest that she actually devised military strategies to drive the danes into Northumbria... I have personally never really believed that she was commanding armies herself, more likely she was consulting with her brother and using Wessex's support to supply Mercian armies with the means to drive the danes out.. But there are some authors who would argue otherwise. After her husband died, there was really nobody in the line of succession as they only had a daughter and no son. Mercia could not afford to have any infighting with the Danes still breathing down their necks, and realistically she had the support of Edward of Wessex. No Mercian lord at the time could afford to make an enemy of Wessex. Edward for his part left his sister to rule, albeit Mercia was basically a vassal of Wessex at this point. So while she was renowned throughout Mercia, she really did nothing without the approval of her brother, the king of Wessex. Again to be fair, there is some debate about this. Supporting the evidence that she was a Vassal to Edward, as I believe she was, Her daughter took power briefly after her death, but shortly after Edward of Wessex took full control of Mercia and Aethelflaeds daughter was taken back to Wessex.
She is an extremely interesting character during an extremely interesting time in british history.
TL;DR: She was the eldest child of Alfred, and brother to Edward. Half of Mercia was occupied by danes, she came to power during a time of weakness among Mercian lords and with her brothers help, drove the danes from Mercia. While she ruled Mercia, she was a vassal to her brother in Wessex. Came to power through her husband when they had no sons and was beloved by most for her shrewd leadership and air of fairness. may or may not have been a military leader.
Robin Fleming, Britain after Rome: The Fall and the Rise, 400 to 1070
edit. Edited a few things for clarity.
It's also important to note that period was far more egalitarian than most people believe in regards to women. They were allowed to divorce and keep custody of children afterwards, were allowed to inherit, and to own property separate from their husband. One woman left everything to her maid instead of her son, something he took her to court over and lost.
The idea of a sole male heir of property wasn't the status quo, and women often married who they pleased with access to a wide variety of different vocations. It wasn't until the Normans came that a true gulf happened between the sexes and became what we think of it today.
I feel this is important to understand why people might embrace a female ruler at all as opposed to later centuries where it would be nearly impossible.