In 1537, he was one of the ambassadors sent to the French Court to negotiate the marriage of James to Mary of Guise, whom he espoused as proxy for the King.
At the time of Flodden, Maxwell was admiral of a fleet which was proposed to go to France, but which on the voyage was driven back and only arrived at Kirkcudbright on the day after the battle.
He was in company with the king at the Battle of Melrose on 25 July, when an unsuccessful attempt was made by Walter Scott of Branxholme and Buccleuch to get possession of him.
Upon the escape of the king from Falkland Palace to Stirling in July 1528, Maxwell separated himself from the party of Angus and was chosen a member of the new council.
Having accompanied the king to Edinburgh, he was again made lord provost of the city and, on 26 August, frustrated an attempt by Angus to take possession of it.
The execution of John Armstrong, who was partly under his protection, was especially distasteful to Maxwell, but he afterwards became reconciled to the king, and on 17 November 1533 was appointed an extraordinary lord of session.
After the death of the king's first wife, Madeleine of Valois, Maxwell was sent in December 1537 with other ambassadors to conclude a treaty of marriage with the recently widowed Mary of Guise.
He joined the army which assembled on the Borough Muir of Edinburgh in October 1542 and, having in vain urged that battle should be given to the English, after its disbandment he took the principal part in raising a force for a new expedition.
The captive nobles were permitted to return to Scotland upon paying a ransom and entering into a bond to aid the English king, by force if necessary, in his scheme for a marriage of Prince Edward with the young queen, Mary Stuart.
Maxwell was at Carlisle in March 1543 and was able to write to the King's widow, queen Mary of Guise, to arrange the passage of her French servants who took messages on to the Duke of Suffolk at Newcastle.
Maxwell showed his hostility to Beaton by proposing, and getting passed, an act that all should have liberty to read the Bible in the Scots and English tongues.