The seventh earl was a man of great vigor. He cleared many of his estates from encumbrances, and became an early favourite of Charles I. In December 1626 he was appointed a member of the privy council of Scotland and a commissioner of exchequer. On the death of his kinsman, John, earl of Montrose and president of the council, the office was immediately conferred by the king on Menteith (January 1628), and on 16 May 1631 confirmed to him for life. In July 1628 he was created justice-general of Scotland, and the king, consulting Menteith on everything relating to Scottish affairs, obliged him frequently to travel up to his court at London, and made him a member of the privy council of England. He gave him an annual pension for life of 500l., and promised him a further gift of 5,000l. sterling as soon as the condition of the royal treasury permitted its payment. But this was never paid.
Something like a genealogical craze took possession of the Scottish nobility at this period. Menteith shared in the rivalry, and having ascertained his descent from Eufamia, countess palatine of Strathearn, and granddaughter of Robert II, by the advice of Sir Thomas Hope, king's advocate, he resolved to pursue his claim to that earldom. Menteith proposed to renounce formally his claim to some of the lands of the earldom which were annexed to the crown, but sought to recover others in possession of subjects. The king not only consented to what was proposed, but gave Menteith 3,000l. sterling for the renunciation. He also granted him a patent, on 31 July 1631, creating him and his heirs earls of Strathearn, and ratified in his favour the old charters which had been granted to his ancestor, David, earl of Strathearn.
These claims were later disputed by William's enemies, he was stripped of some of his titles and lands, but later on had them restored by this King, who needed his aid against his own enemies.
Source www.thepeerage.com from The Dictionary of National Biography, Oxford Univ. Press
http://onlinebooks.library.upenn.edu/webbin/metabook?id=dnb