The eastern terminus of Great Western Highway is at Railway Square, at the intersection of Broadway with Quay Street, in the inner-city suburb of Haymarket and just south of the Sydney CBD.

Parramatta Road became one of the colony's most important early roadways, and for many years remained one of Sydney's premier thoroughfares.

[5] In 1813, acting on the instructions of Governor of New South Wales Lachlan Macquarie, Gregory Blaxland, William Lawson and William Wentworth led an 1813 expedition that travelled west from Emu Plains and, by staying to the ridges, were able to confirm the existence of a passable route directly west from Sydney across the Blue Mountains.

From this point they were able to see that the worst of the almost impenetrable terrain of the Blue Mountains was behind them, and that there were easy routes available to reach the rolling countryside they could see off to the west.

Macquarie then despatched Surveyor George Evans to follow Blaxland, Lawson and Wentworth's route and to push further west until he reached arable land.

This route turned north 2 kilometres (1.2 mi) south of O'Connell to run northwest to where Kelso is now located, then west across the Macquarie River into Bathurst.

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.

Mitchell was also concerned to improve the worst sections of the road, which were the climb from the Cumberland Plain, on which Sydney sits, and the descent of Mount York, down the western side of the Blue Mountains.

In improving the eastern ascent Mitchell adhered largely to Cox's route, which follows the southern side of an east-falling gully to reach the plateau at where Blaxland is now located.

The Mitchell's Pass and Lennox Bridge served as the main route to the Blue Mountains for 93 years until 1926, when the Great Western Highway was re-routed via sections of the former Lapstone Zig Zag including the Knapsack Viaduct.

[10] After protracted arguments first with Governor Ralph Darling and then his successor Richard Bourke, and ignoring orders, Mitchell surveyed, designed and had built what is now known as Victoria Pass, where the highway drops from the Blue Mountains into the Hartley Valley.

Midway down the road had to be supported on a causeway formed by massive stone buttressed walls, where a narrow ridge connects two large bluffs.

From Marrangaroo a new road was built westward, running south of Wallerawang to meet Mitchell's 1830 deviation immediately east of Mount Lambie.

Its eastern end was extended to the intersection of Broadway and Wattle Street on 22 February 1967 (part of Sydney's Ring Road 1 at the time).

[4] The first recorded major improvement to the route of Great Western Highway was the construction in 1806 of ten bridges along Parramatta Road.

In attempts to improve the gradient of the descent from the Blue Mountains plateau to the floor of the Hartley Valley, Lawson's Long Alley was opened in 1824.

Originally the Great Western Road crossed the Nepean River at Penrith by means of a ferry adjacent to the Log Cabin Hotel.

They were in turn replaced in the mid-1950s to obtain the necessary height clearances for overhead wiring for the electrification of the Main Western Railway from Penrith to Bowenfels.

However rapid improvements in motor vehicle performance meant that in 1920 Victoria Pass was rebuilt to become the main route again.

After the ascent of the eastern escarpment by the Main Western Railway was deviated for the second time in 1913 to its current route via Glenbrook Gorge, the Great Western Road was also deviated at this point for a second time in 1926 by the then Main Roads Board, which rerouted it via the disused 1867 stone arch railway viaduct across Knapsack Gully and around the southern side of Lapstone Hill to gain the first plateau in the ascent of the Blue Mountains.

The viaduct was closed to motor traffic when the M4 Motorway was extended west from Russell Street to connect to the highway at Lapstone in 1994.

In 1957 a short deviation immediately west of Linden eliminated two narrow overpasses of the railway, both of which had right angle bend approaches from both directions.

The fact that this section of the highway is on the southern side of a very high escarpment poses severe ice problems during winter, due to the lack of sunlight.

In June 1993 the highway route was severed at Emu Plains with the closure to road traffic of the Knapsack Gully Viaduct.

The portion of Great Western Highway west from Russell Street to Mitchells Pass Road is now only used by local traffic to access residential properties.

A major realignment west from Mount Boyce (the highest point on the highway) to eliminate the Soldiers Pinch and other nearby sharp curves was completed in 2002.

From Railway Square to Woodville Road, the highway was widened to its present width when it was reconstructed in reinforced concrete in the 1930s.

Between 1981 and 2015 the NSW Government Roads & Maritime Services duplicated the highway between Leonay and 1 kilometre (0.62 mi) west of Katoomba, generally working westward.

This was completed as follows: Four-lane undivided sections are the 2.5-kilometre (1.6 mi) of Victoria Pass (except for Mitchell's 1832 causeway, which is two lanes), and from Boyd Street in Kelso to the terminus of the highway at its junction with the Mitchell and Mid Western Highways in West Bathurst (parts have a raised median strip).