At Galashiels Johnston studied French, German, and Spanish and described the efforts of his languages teacher as a possible inspiration for joining the Foreign Office.

[2] Johnston then attended the University of Glasgow, graduating with an MA (Hons) in Political Science and Government.

[3] Johnston described negotiating on behalf of the European Union on the language on the responsibility to protect, adopted by the UN World Summit in 2005, as a personal highlight of his career.

[3] He served as the British Ambassador to the European Union's Political and Security Committee from 2017 to January 2020.

[6] On 5 March 2021, the Irish Times published a letter from Johnston, setting out the rationale for the decision of the British government to unilaterally extend the grace period for post-Brexit checks on some goods entering Northern Ireland from Great Britain.