During the First Anglo-Sikh War, Lumsden served as a commander of a company in his regiment. At the Battle of Sobraon he was part of the storming division on Hugh Gough's left flank. During the escape of the Sikh cavalry, Lumsden was shot in the foot which left him with a slight but permanent limp for the rest of his life.[1]

At the conclusion of the war, Lumsden was hand-picked by Sir Henry Lawrence to become one of his assistants, a band of chosen men who would become known as Henry Lawrence's "Young Men". A number of these assistants had been brought to Lawrence's attention on the battlefield at Sobraon, including John Nicholson, William Hodson and Herbert Edwardes. Lawrence had been a friend of Thomas Lumsden, serving with him as an artillery officers earlier in his career. Lumsden later claimed that what first brought him to the attention of Lawrence was an incident a year or two previously, when he and a subaltern had been held prisoner by a gang of robbers following a shooting expedition on the banks of the Sutlej. A servant smuggled them a pencil and paper allowing them to call for help; help coming in the form of a cavalry led by Lawrence that same evening.[1]

Lumsden's first posting was to Kangra in 1846, where he served out the summer. He was thereafter summoned to Lahore. Whilst in Lahore, he became privy to the politics of the Lahore Durbar, at the time dominated by the Maharani Jind Kaur and her lover Lal Singh.[1] Noting how everyone of the Sirdars made no secret of their dislike for Lal Singh, he concluded that they were anxiously awaiting the time when the British would withdraw, and Lal Singh would consolidate power. In response to the political instability, like many fellow offers, but in contrast to his superior, Lawrence, Lumsden favoured the annexation of the Punjab.[1]

The sale of Kashmir to Gulab Singh had created considerable unrest amongst Kashmiri locals. In late 1846, Lumsden accompanied an expedition led by Lawrence to dispose the incumbent governor Imam ud-Din, who had the backing of Lal Singh and support the new rule of Gulab Singh.[1] On his return to Lahore, Lumsden was given a mission to command a force of Sikh infantry to reconnoitre the hills of Hazara. Despite initial concerns of leading men he had formerly seen as the enemy, and noting their fondness for opium, he would remark of his Sikh sepoys that "they were first rate men, ready for any work, always in the best of humours, fond of their officers, and just as obedient to orders as our own troops, and not giving one quarter the trouble as the latter do."[1] Lumsden's mission was a success and he received the thanks of the government, and what he prized more, the approbation of Henry Lawrence.[1]