John Churchill was born to Elizabeth and Winston Churchill (the more famous holder of that name is a descendant) in the immediate aftermath of the English Civil War. His father had fought for the Royalists during the conflict and had suffered badly for it -- they lived in very modest circumstances until the end of the Commonwealth. His staunch support for Charles II paid off with the return of the king, however, and one of the fruits of this was the appointment of 17-year-old John Churchill to the household of the man second in line to the throne, then-Lord High Admiral, the Duke of York. Joining the navy, he remained at court for a while, but saw turns of duty first in the Mediterranean and in the last of The Dutch Wars. In the latter, he became a Marine Officer, and was attached to the siege of Maastricht, his first taste of land battle. Having come to the attention of the French crown during this action, he moved onto the fighting in Alsace, where he fought with one of the leading generals of the day, Marshal Turenne.
That war ended in 1674, and Churchill settled down to peaceful life -- for a while. In 1678, he married Sarah Jennings, who would come to fiercely present his interests at court while he was off fighting on the Continent. In the years to follow he engaged in various diplomatic missions to Spain and ex-enemy Holland, largely in opposition to French interests. In 1685, Charles II died without issue, and Churchill's former employer, the Duke of York, became James II. Shortly thereafter James elevated him to the peerage as Baron Sandridge.
Within a few months, the new king faced a series of rebellions, one of which was by the Duke of Monmouth. Churchill was appointed head of the loyalist troops, then quickly subordinated to the Earl of Feversham. It is believed that this lack of confidence was what eventually turned Churchill from loyalty to the Stuart kings. Churchill nevertheless distinguished himself during the fighting, and became an important figure in the army.
In 1688, William of Orange invaded England with the support of most of the nobility, as James II was a Catholic and appeared to be on the road to introducing absolutist rule into his kingdom. Churchill deserted to the Orange cause, which caused most of the army to come with him and put James into a very difficult position. He quit the country for France rather than fight. The Glorious Revolution had been pulled off with far less bloodshed than anyone expected, and the Stuarts no longer ruled in Britain. Churchill was named Earl of Marlborough in reward.
Marlborough was out of the public sphere to a large extent for the next few years, as William did not trust the former Stuart supporter entirely. During this time, his most notable activity was some time as the Governor of the Hudson's Bay Company. The Canadian town of Churchill, Manitoba a former Company outpost, gains its name from this connection.
He returned to the forefront with events leading up to the War of the Spanish Succession in 1701. The grandson of Louis XIV was put forward as heir to the throne of Spain, and rather than allow France to expand its power to such a great extent, a coalition of European powers including England prepared for war. Marlborough was first put to use commanding English and Dutch forces opposing the Swedish allies of France. He was successful in this, and when William died in 1702, Marlborough was reaching his peak.
William's successor Anne was a close friend of Marlborough's wife, and he enjoyed the new queen's confidence. The same year, war with France finally broke out into the open. Despite the intransigence of his Dutch troops, Marlborough won a series of victories in 1703 and was named Duke of Marlborough in return. The war continued, however, and in combination with Prince Eugene of Savoy he beat the French at the Battle of Blenheim and Battle of Ramillies.
Things were taking a turn for the worse in Spain, however, and his wife's friendship with the queen was gradually falling apart. Despite the earlier defeats, the French refused to give in, and continued pounding on Belgium. By 1708 he had had to fight a pitched battle against French forces once again, this time winning the Battle of Oudenarde. One more win followed at the Battle of Malplaquet in 1709. In Britain, however, the Whig Party had come to power and continued undermining the relationship he had with Anne. By 1711 he was recalled from the Continent, and sent into a brief exile. The war ground on and eventually ended in mutual exhaustion in 1714.
Anne died shortly thereafter, and once again Marlborough backed the right horse, putting up funds in support of the Elector of Hanover. The Elector became king as George I, and Marlborough returned to England and settled into retirement. He spent most of his remaining days working on his new home, Blenheim Palace, and died in 1722.