The 1707 Union combined Scotland and England into the Kingdom of Great Britain but the two countries retained separate legal systems.
Reconciling the two led to an increase in legal work; Forbes was admitted to the Scottish Faculty of Advocates on 26 July 1709, a professional body with less than 200 members in 1714.

Like their father, the Forbes and their associates were political allies of John Campbell, 2nd Duke of Argyll; from 1713 to 1727, John Forbes  was MP for Inverness-shire, then Nairnshire, both controlled by Argyll.
Other members of this circle included Duncan's brother-in-law Hugh Rose (1684-1755) and his cousin George (1685-1765), who from 1741 to 1747 was MP for Ayr Burghs, another constituency controlled by Argyll.

In 1714, Forbes was appointed Sheriff-depute for Edinburghshire and Deputy lieutenant for Inverness-shire.
During the Jacobite Rising of 1715, his patron Argyll was government commander in Scotland; the Forbes brothers raised a number of independent companies and fortified Culloden and Kilvarock.
They joined forces with Lord Lovat and captured Inverness, just before the Rising ended at the Battle of Sheriffmuir.
In recognition of his services, Forbes was made Depute-Advocate in March 1716.

This required him to prosecute Jacobite prisoners, many of whom had been moved to Carlisle for trial.
Forbes considered this unfair, as it was contrary to the accepted practice they be tried in the counties where the actions were alleged to have taken place.
He allegedly collected money for their support and also wrote to Sir Robert Walpole recommending clemency, which led to accusations he was pro-Jacobite.
This did not affect his career and in 1721, he became MP for Ayr Burghs; in 1722, he was elected for Inverness Burghs, which he held until 1737.

Forbes was appointed Lord Advocate in 1725, an office increased in importance by the suspension and later abolition of the position of Secretary of State.
He was almost immediately involved in the 1725 malt tax riots, caused by protests against a new tax that increased the price of beer.
These affected many cities, the largest in Glasgow, where rioters sacked the house of Daniel Campbell, MP for Glasgow or Clyde Burghs, who voted for the tax in Parliament.
Forbes ordered the arrest of several Glasgow magistrates suspected of inciting the unrest; they were soon released and the government made a number of concessions, although Daniel Campbell was awarded £6,080 in compensation.

Following the 1737 Porteous Riots in Edinburgh, a bill was introduced in Parliament imposing penalties on the city, which was opposed by Argyll and Scots MPs in the Commons, Forbes included.
His speeches of 16 May and 9 June on this topic were his last in Parliament; on 21 June, he resigned as an MP and took up the position of Lord President of the Court of Session, becoming the senior legal officer in Scotland.
The 1707 Union combined Scotland and England into the Kingdom of Great Britain but the two countries retained separate legal systems. Reconciling the two led to an increase in legal work; Forbes was admitted to the Scottish Faculty of Advocates on 26 July 1709, a professional body with less than 200 members in 1714.
Like their father, the Forbes and their associates were political allies of John Campbell, 2nd Duke of Argyll; from 1713 to 1727, John Forbes  was MP for Inverness-shire, then Nairnshire, both controlled by Argyll. Other members of this circle included Duncan's brother-in-law Hugh Rose (1684-1755) and his cousin George (1685-1765), who from 1741 to 1747 was MP for Ayr Burghs, another constituency controlled by Argyll.
In 1714, Forbes was appointed Sheriff-depute for Edinburghshire and Deputy lieutenant for Inverness-shire. During the Jacobite Rising of 1715, his patron Argyll was government commander in Scotland; the Forbes brothers raised a number of independent companies and fortified Culloden and Kilvarock. They joined forces with Lord Lovat and captured Inverness, just before the Rising ended at the Battle of Sheriffmuir. In recognition of his services, Forbes was made Depute-Advocate in March 1716.
This required him to prosecute Jacobite prisoners, many of whom had been moved to Carlisle for trial. Forbes considered this unfair, as it was contrary to the accepted practice they be tried in the counties where the actions were alleged to have taken place. He allegedly collected money for their support and also wrote to Sir Robert Walpole recommending clemency, which led to accusations he was pro-Jacobite. This did not affect his career and in 1721, he became MP for Ayr Burghs; in 1722, he was elected for Inverness Burghs, which he held until 1737.
Forbes was appointed Lord Advocate in 1725, an office increased in importance by the suspension and later abolition of the position of Secretary of State. He was almost immediately involved in the 1725 malt tax riots, caused by protests against a new tax that increased the price of beer. These affected many cities, the largest in Glasgow, where rioters sacked the house of Daniel Campbell, MP for Glasgow or Clyde Burghs, who voted for the tax in Parliament. Forbes ordered the arrest of several Glasgow magistrates suspected of inciting the unrest; they were soon released and the government made a number of concessions, although Daniel Campbell was awarded £6,080 in compensation.
Following the 1737 Porteous Riots in Edinburgh, a bill was introduced in Parliament imposing penalties on the city, which was opposed by Argyll and Scots MPs in the Commons, Forbes included. His speeches of 16 May and 9 June on this topic were his last in Parliament; on 21 June, he resigned as an MP and took up the position of Lord President of the Court of Session, becoming the senior legal officer in Scotland.
Duncan Forbes of Culloden (judge, born 1685)