He entered Harrow School, under Dr. Drury, at the age of ten, having for fellow scholars Lord Byron and Sir Robert Peel.
Sinclair returned to England, and in 1811 succeeded his father in the Whig interest as Member of Parliament (MP) for the county of Caithness, which he represented at intervals for many years.
In the House of Commons Sinclair formed a close friendship with Joseph Hume and Sir Francis Burdett.
While a member of Parliament Sinclair found time to attend the Edinburgh lectures of Dr. Hope on chemistry, of Dr. Knox and Dr. Monro on anatomy, and also a course on botany.
He supported the Reform Bill of 1832, and in the same year he attracted public attention by refusing William IV's invitation to dine with him on a Sunday.
In 1835 he joined the new 'constitutional' party of Edward Smith-Stanley and Sir James Graham, who had seceded in 1834 from the government of Earl Grey.
He took an active part, already a Conservative, as chairman of Sir Francis Burdett's committee in the famous Westminster election of 1837.
At this time a writer in Blackwood characterised him as "one of the manliest and most uncompromising of the constitutional members of the House of Commons; a friend to the church, the king, and the people."