It seems that the ancestral Crawley was ennobled in the mid 18th century. I guess that before then the family belonged to the gentry, the knight class. Perhaps he who became the first earl achieved some distinction in the military, the Seven Years War? Perhaps he was a wealthy man among the merchants who purchased a patent of nobility (although earl seems like quite a leap). At any rate, the title does not extend back to the middle ages or even to the time of Henry VIII--even though Lord Grantham insists that there had been no Catholic Crawleys since the Reformation.
My first guess was that the Crawleys performed some service in the Restoration (that large painting of Charles II having offered the suggestion), but this seems not to be the case. Is there any information at all on how the Crawleys gained their title?