[2] The first road up the eastern slopes of the Blue Mountains, built by William Cox (1814–15), was in Governor Macquarie's words "pretty steep and sharp" and was found to be subject to serious washways.

When Major Thomas Mitchell was appointed as Surveyor General in 1828, one of the first matters to which he turned his attention was the improvement of the Great Western Road.

Mitchell's attention was focussed on providing a more direct and easily graded route for the Great Western Road.

'And on 1 October 1832, only seven weeks after his arrival in the colony, Lennox was appointed (subject to London's confirmation) Sub-Inspector of Bridges in New South Wales.

By early November 1832, he and his selected twenty convicts; with suitable experience had opened a quarry near the creek, had cut a number of stone blocks and were ready to start building operations.

It replaced an earlier crossing of the creek 600 metres (2,000 ft) further south, which today survives as a walking track.

The success of the railway after 1867 diverted much transport business away from the road until the twentieth century and advent of the motor vehicle.

[4] The bridge served the main route to the Blue Mountains for 93 years until 1926, when the Great Western Highway was re-routed via the Knapsack Viaduct.

[1][9][11] Lennox Bridge spans the deep gully of Lapstone Creek at the head of the steepest part of Mitchell's Pass.

The arch on the south side has a keystone bearing the inscription "DAVID LENNOX", on the north side, "AD 1833", the packing of the bridge to either hand of the main ashlared section is of coursed rubble revetments, making the total length of the bridge carriageway of almost 14 metres (47 ft).

During these repairs new concrete supports were inserted to bear the weight of a new carriageway utilising steel girders to take the traffic then still using Mitchell's Pass.

There is a small amount of car-parking space beside the bridge and a concrete stairwell and path down to the creek on the west side.

The Department of Main Roads decided that the stress had to be removed from the original arch and reconstructed the carriageway so that it was supported on two new concrete walls and borne on horizontal steel girders.

It commands significance respect as the design of a newly arrived Scottish stone-mason, David Lennox, handpicked by the percipient Surveyor General Thomas Mitchell and as the work of some twenty diligent, efficient, tightly supervised and technically able convicts.

[4] Lennox Bridge, Blaxland was listed on the New South Wales State Heritage Register on 2 April 1999 having satisfied the following criteria.

It commands respect as the design of a newly arrived Scottish stone-mason handpicked by the percipient Surveyor General Thomas Mitchell and as the work of some twenty diligent, efficient, tightly supervised and technically able convicts.

Prior to arriving in the Colony he had spent seventeen years on public works executed at the expense of the British Government.

[13][4] The place is important in demonstrating aesthetic characteristics and/or a high degree of creative or technical achievement in New South Wales.

[4] The place has potential to yield information that will contribute to an understanding of the cultural or natural history of New South Wales.