[2] After attending private schools until age 16, Thomson entered the family firm at Saint Petersburg.

In 1830 he joined Earl Grey's government as Vice-President of the Board of Trade and Treasurer of the Navy, an office he held until 1834.

In November 1831 Thomson accompanied Lord Durham to Paris to negotiate a new commercial treaty with July Monarchy France, but the project was not accomplished.

[3] Sydenham was just as anti-French as Lord Durham had been, and he encouraged British immigration to make the French Canadian population less significant.

[1] In addition to this measure he carried another for local government, and he set on foot improvements in the matters of emigration, education, and public works.

Charles Greville, in his Memoirs wrote about Thomson: In spite of his vanity he had many admirable qualities: tact, judgment, and prudence, firmness and decision, indefatigable and well-ordered application, and, above all, a disinterested devotion to the service of his country.

[6] Sydenham wanted to make Canada more financially independent, so that there would less danger of annexation by the United States.

He had been working on this policy throughout the 1830s, when he was President of the Board of Trade in Britain, though he had little time to implement any economic reforms once he had arrived in Canada.

However, on 4 September, Sydenham was riding a spirited horse near Parliament House, but could not, for a long time, get the animal to pass that building.

After a severe application of spur and whip, however, the horse proceeded, but immediately after, put his foot upon a large stone ... not being able to recover, fell and dragged his rider with him, fracturing the leg, and lacerating it above the knee.

The Old Sydenham Heritage Conservation District, in the southeastern sector of Downtown Kingston, was formally designated by the city council on 24 March 2015.