Proud and quick-tempered, Johnston became notorious as a duelist and debater. A quarrel with the Anglican hierarchy resulted in his emerging as a leader of Nova Scotia's Baptist community. Dissenter credentials, along with links to Halifax's business elite, enabled Johnston to move into politics. For more than a decade he led the moderates who sought compromise between Tory oligarchy and Liberal democracy.
Ousted from power in 1848 by Joseph Howe, Johnston continued as leader of the Conservative Party. Restored to the premiership in 1857, Johnston soon moved to the bench, allowing Charles Tupper to become his successor. A persistent critic of colonial democracy, Johnston advocated Confederation as a remedy for the alleged failings of Responsible Government. His reward came in 1873 when Ottawa named him Nova Scotia's lieutenant-governor, but ill health and then death frustrated the appointment.