He left England in August 1688, and returned in November with William of Orange’s expedition He was rewarded with appointment as envoy to Prussia from 1690 to 1692, taking to the Elector of Brandenburg a gift of the Order of the Garter.

[1] Johnston returned to the front rank of Scottish politics with the ‘New Party’ experiment of 1704, when he entered office along with Tweeddale.

[1] At the 1708 British general election Johnston was returned as Member of Parliament for Ilchester on the Tory interest of his wife's family.

He absented himself from the Tory attack on the ministry’s handling of the abortive Jacobite invasion of 1708 although he did vote against the impeachment of Dr Sacheverell in 1710.

He procured a lease in 1702 and commissioned the architect John James to plan and erect a mansion (known after 1813 as Orleans House) – a project which spanned the following 35 years.

"[2] A baroque octagonal room, designed by architect James Gibbs, was added in 1720 for entertaining Caroline who regarded Johnston with great favour.

In 1716 he married as his third wife Lucy Claxton, daughter of Thomas Claxton of Dublin, sister of Frances Countess of Rosse and Viscountess Jocelyn, sister of Mary Carter, cousin of Edward Lovett Pearce, cousin of Sir John Vanbrugh and great-granddaughter of Sir Dudley Carleton of Imber Court Surrey, nephew and heir of Viscount Dorchester.

It was said George I "often conversed with him very familiarly" and that Johnston was "a great favourite of Queen Caroline, who was much entertained with his humour and pleasantry".

It was also said "he keeps out a very great rank, and frequently has Mr. Walpool and the greatest courtiers with him at his country house near London; and the King sometimes does him the honour to dine with him".

His eldest daughter, Lucy, who was baptised at Twickenham on 7 July 1717, married General George Preston, Colonel of the Scots Greys.