On 24 July 2019, Wallace was appointed Secretary of State for Defence by Prime Minister Boris Johnson, succeeding Penny Mordaunt. In August 2019, he was overheard discussing Prime Minister Johnson's controversial prorogation with Florence Parly, the French Minister of Armies. Wallace suggested that the reason for the prorogation of parliament for five weeks was to prevent MPs from blocking the government's Brexit plans rather than the government's official position that it was to introduce new legislative agenda. The government responded to his comments by stating that he had "misspoken". This prorogation was later deemed unlawful by the Supreme Court on 24 September 2019.