Originally the value of the guinea was  twenty shillings, but its value changed during the course of its life due to the relative value of gold and silver. In 1670 the weight of the coin was reduced from 8.4–8.5 g to 8.3–8.4 g, but the price of gold continued to increase, and by the 1680s the coin was worth 22 shillings.

The value of the gold Guinea was stabilised during William III’s reign in 1696 through devaluation of the gold coinage and restoration of silver coinage. The coin to the left is a 1701 William III Guinea. It was for sale in my eBay shop and sold for £1750 in June 2009. William and Mary had joint sovereignty over the Kingdom of England, as well as the Kingdom of Scotland.  King William III and his wife Queen Mary II, a daughter of James II. Their joint reign began in February, 1689 the year this halfcrown was issued. They were called to the throne by Parliament, replacing James II, who was "deemed to have fled" the country in the Glorious Revolution of 1688. After Mary died in 1694, William of Orange ruled alone until his death in 1702. Their rule was the only period in British history in which "joint sovereigns" with equal powers were allowed to reign; usually, the spouse of the monarch has no power and is simply a consort. Note King Williams hooked nose which can be more clearly seen on the coin.

The value of the Guinea deteriorated again during the reign of George III because very little silver or copper coin was minted. In fact most of the silver in Europe had already been mined out and was in very short supply. The coin to the left is gold Guinea graded graded EF 70 by CGS and valued by them at £1250. George III's long reign was marked by a series of military conflicts involving his kingdoms, much of the rest of Europe, and places further afield in Africa, the Americas and Asia. Early in his reign, Great Britain defeated France in the Seven Years' War, becoming the dominant European power in North America and India.  However, many of its American colonies were soon lost in the American Revolutionary War, which led to the establishment of the United States of America.  A series of wars against revolutionary and Napoleonic France, over a twenty-year period, finally concluded in the defeat of Napoleon in 1815.

The guinea was finally replaced in 1816 by the Sovereign.

Until the introduction of the guinea, coins had mostly been hand-stamped, where a coin blank would be placed between a pair of engraved dies and hammered to produce the raised pattern. Some milled coins were made in Cromwell's time but  production of the first guineas coincided with the time when all coins were milled. Produced mechanically, and though some hand stamped guineas were produced in the first couple of years, all others were mechanically produced.