Here he gained honours in Persian, Hindustani, and Hindi at the company's college, and in September of the same year was appointed to a post under Edward Thornton at Saharanpur.

On the annexation of the Punjab after the Second Anglo-Sikh War in March 1849, he was appointed to take part in the administration of the new province, and was sent by Sir Henry Lawrence, together with Colonel Marsden, as deputy-commissioner over him, to Pakpattan.

After the capture of Delhi he was one of the special commissioners appointed to hunt up the rebels, and in this capacity was principally engaged in examining the papers of the Nana of Cawnpore.

Lord Mayo approved and authorised him to proceed to England, and if possible to St. Petersburg, with the object of arranging with the Russian government a definition of the territories of the Amir of Kabul.

At this time Yakub Beg, as the separatist anti-Chinese leader of Yarkand and Kashgar, in order to establish relations between the territories occupied by his army and India, had sent an envoy to the viceroy with the request that a British officer might be deputed to visit him.

The journey from Lahore to Yarkand and back, a distance of two thousand miles, was accomplished in six months; but the expedition failed to produce all the results expected from it, because of the absence of the amir from his capital on its arrival.

Of the part of the journey over the high table land (16,000 feet (4,900 m) above sea level) connecting the Karakoram and Kuen Luen ranges, Forsyth wrote that the influence on him was so great: "a good breath, even when the body was in a state of rest, was a luxury seldom enjoyed ... and a feeling of exhaustion and severe nausea were continuous.

His powers on this occasion seem not to have been sufficiently defined, and Lambert Cowan, the then commissioner of Ludhiana, had anticipated his arrival by executing many of the rebels, a course of action which, though contrary to instructions, Forsyth felt bound to support.