While he was governor of Trinidad and Tobago, the Trinidad Government Railway was extended from Cunupia to Tabaquite. The railway junction formed by this extension was named Jerningham Junction. He also instituted an award for the top student in the Island Scholarship examinations, a gold medal called the Jerningham Medal (today known as the President's Medal).
Jerningham advised the Colonial Office to suspend the charter of the Borough of Port of Spain in 1898 following a dispute with the Borough Council. The borough charter was suspended on January 1 1899; the suspension remained in effect until 1914. This action made Jerningham very unpopular with the population.