In 1813 Lawson, with Gregory Blaxland and William Wentworth, led an expedition westwards from Sydney to cross the Blue Mountains, which formed a barrier to further expanion of settlement and had been thought to be impassable.
They departed on 11 May 1813 with four servants, five dogs and four horses. The route they traversed is, with some slight variations, still the one used by travellers today. On the 31 May they reached Mount Blaxland from where they could see the plains to the west. Beyond the mountains the explorers found great expanse of open country, which they surveyed.
In 1819 Governor Lachlan Macquarie established a new settlement, Bathurst, in this new country, and appointed Lawson as commander of the Bathurst settlement, a position he held until 1823. From Bathurst Lawson explored as far north as Mudgee.
After leaving the Army Lawson was a member of the New South Wales Legislative Council from 1843 to 1848. He died at Prospect, New South Wales, in 1850. The town of Lawson in the Blue Mountains is named for him.