Promoted to rear-admiral on 5 February 1755, Holburne was quickly sent in May to reinforce Admiral Edward Boscawen's fleet in the North Atlantic. Boscawen's mission to intercept French ships from resupplying Canada was in danger as his ships' companies were not only sickly but short of men. Holburne was later appointed a member of the court-martial that was convened to try Admiral John Byng. The trial began in December 1756 and ran until late January 1757. When members of the tribunal requested to be relieved of their oaths before the House of Lords to save Byng, Holburne refused. "...all the court martial seemed terrified...except old Admiral Holbourne, who cursed and swore at the bar of the House, because Byng was not shot out of the way, without giving him the trouble of coming from Portsmouth".