He further received from King David a charter of the lands of Merton and Merchiston, bearing date at Edinburgh, the 11th February 13581 (confirmed in 1363), and the same year proceeded to the Continent in gallant array.
Under 1358 Tytler has: - "At this period the Scottish kingdom was beset with designs against its independence, so dangerous in their nature and so artfully pursued that it was unfortunate that a spirit of military adventure carried many of its best soldiers to the continental wars. Sir Thomas Bisset, and Sir Walter Moigne, with Norman and Walter Leslie, previous to King David's return had left the country on an expedition to Prussia, in all probability to join the Teutonic knights who were engaged in a species of crusade against the infidel Prussians. Not long after Sir William Keith, Marshal of Scotland, SIR WILLIAM SINCLAIR, Lord of Roslin, Sir Alexander de Lindesay, Sir Robert Gifford, and Sir Alexander Montgomery, each with a train of sixty horse and a strong body of foot soldiers, passed through England for the continent eager for distinction in foreign wars, with which they had no concern, and foolishly deserting their country when it most required their services. (Rotuli Scotiae, volume 1, 32 Edward III, page 380) Yet this conduct was more pardonable than that of the Earl of Mar, who entered into the service of England, and with a retinue of 24 knights and their squires, passed over to France in company with the English monarch and his army. The example was infectious, and the love of enterprise, the renown of fighting under so illustrious a leader, and the hopes of plunder, induced other soldiers to imitate his example". If William St.Clair of Roslin is the same as Willielmus de Sancto Claro, Dominus de Pentland, who attests a charter granted by Patrick de Graham, Lord of Kinpont, and David de Graham, Lord of Dundaff, in the King's presence at Edinburgh, 1362, [from Hay's Genealogie] then that must be accepted as evidence of his safe return from the Continent; and indeed there is a record of William de St.Clair, Lord of Dyserth, attesting a royal charter at Dundee, 20th November 1364, while in the sheriffship accounts for Inverness in January 1367, there is noted in the hands of Sir William Keith, Knight, "Wardam hed. Willi. de Sco.Claro, / p. excambin, l.e. de Anno Redd. Coital catanie £ 10".
This William St.Clair married [Diploma of Succession] Isabella, second daughter, and eventually sole heiress, of Malise II of Orkney and Caithness, and V of Stratherne and had issue -
It is thought that William St.Clair died leaving his sons in minority, for though it is stated [Hay's Genealogie] that his son Henry was sent Ambassador to Denmark in 1363, it is certain that Henry was not Earl till 1379, and presumably younger than his cousin Alexander de Ard, who, a minor in 1367, was Governor and Commissioner of Orcadia in 1375. The minority of William de St.Clair's son explains the appointment in 1364 of Thomas de St.Clair as ballivus regis Norvagie, by virtue of his guardianship of Henry St.Clair the Earl prospective.
The next three Lords of Roslin were: -
IV HENRY I, the Holy, 42nd Earl of Orkney;
V HENRY II, 43rd Earl of Orkney, Lord St.Clair; and
VI WILLIAM, 44th Earl of Orkney and the Earl of Caithness, and Lord St.Clair.