Jenkins graduated at Oxford in 1600 and was called to the Bar in 1609. His life falls into two well-marked periods: the period of calm, and the time of storm and stress during the Civil War. Of the earlier period little is known, but his practice must have been extensive for he accumulated a considerable fortune. He had opposed the king's method of raising money and had incurred the displeasure of the bishops, but when war broke out he supported the king firmly and consistently. In 1643 he was appointed judge of the Court of Great Sessions for the Carmarthen circuit, much against his will, because the expenses of the office exceeded the emoluments; it was from this office that he got the title of ‘Judge Jenkins.’ He incurred the particular odium of the parliamentarians by condemning several of them to death for treason, though they managed to escape. Consequently, when he was captured at Oxford in 1645 he was charged with the same offence, but argued, logically enough, that consistent support of the king could not be treason against the king. The charges were ultimately dropped on grounds of expediency, but he enlivened his imprisonment by writing a series of controversial pamphlets that were collected into a volume in 1648. He also completed his Reports of 800 common law decisions, much in the style of ‘Leading Cases.’ He was not finally released from surveillance until the Restoration, and then retired to his estate at Hensol.