John Sinclair, 10th Lord Sinclair was an active Royalist during the Civil War. On June 19, 1633, he was a member of the Parliament held by Charles I of England at Edinburgh. On March 22, 1639, he is recorded, with other Lords, as commanding 1,000 musketeers from Dalkeith to Edinburgh. On June 5, 1644, a committee was appointed to march an army towards England by the Earl of Argyll, Earl of Lothian, Lord Kirkcudbright and Lord Sinclair. On June 11, 1644, he is on the committee for considering what may concern the army in Ireland. William Thomsone was ordered to pay Lord Sinclair 20,000 merks on June 21, 1644. A letter written by Lord Sinclair on behalf of the Royalist James Graham, 1st Marquess of Montrose from Amsterdam was read out in Parliament on May 30, 1650. He was one of the signatories to "The Northern Band and Othe Engagement" sent by John Middleton, 1st Earl of Middleton to General Leslie in 1648.