He was for a time manager in South Australia for the Picturesque Atlas of Australasia, and afterwards was partner with C. G. Gurr in an estate agency and auctioneering business at Adelaide.

A week later the second liberal Holder ministry was formed with Jenkins as chief secretary, and when Holder went into federal politics in May 1901, Jenkins became premier, chief secretary, and minister controlling the Northern Territory, forming government with the support of none other than the Australasian National League (formerly National Defence League).

As premier he took an important share of the work connected with ministerial bills, and among the acts he was responsible for were those providing free education, the Happy Valley water-supply system for Adelaide, and the transcontinental railway.

He also played a major role in an agreement between the States about the River Murray, and in continuing attempts to develop the Northern Territory.

In 1918 he stood for Putney in an election for the British House of Commons, on behalf of the short-lived right-wing National Party, but was defeated.

He had a good standing in the city of London, and when the chamber of commerce sent a delegation to the United States of America, Jenkins was the chief spokesman.