He was a member of the Commission on the Union before 1707 but acted as an informant to his Jacobite colleagues and later wrote an anonymous memoir of its dealings.

He developed the estate and exploited the coal reserves to become one of the wealthiest of Scottish commoners, and gathered a major electoral interest in Lanarkshire and Midlothian.

He was attracted to a political alliance in the hope of achieving legal toleration for Episcopalians but as this was not forthcoming, he went into opposition, and remained disaffected with the Court for the rest of his time in the Scottish parliament.

[2] After the union Lockhart he decided to seek election to Westminster to serve the Jacobite cause, and to distract the ministry from suspicion about the intended invasion.

He was imprisoned in Edinburgh Castle, but probably, through the favour of John Campbell, 2nd Duke of Argyll, he was released without being brought to trial; but his brother Philip was taken prisoner at the Battle of Preston and condemned to be shot, the sentence being executed on 2 December 1715.

[5] Argyll's influence was again exerted in Lockhart's behalf, and in 1728 he was permitted to return to Scotland, where he lived in retirement until his death in a duel on 17 December 1731.

Lockhart was the author of Memoirs of the Affairs of Scotland, dealing with the reign of Queen Anne up to the union with England, first published in 1714.

[6][4] Lockart was the source of intelligence revealing the extensive bribery of Scottish parliamentarians prior to the Treaty of Union, giving rise to the famous Robert Burns line: "bought and sold for English gold".