Drive most of S.R. 120 today and you encounter red lights, strip malls, subdivisions and traffic, a far cry from its muddy beginnings. S.R. 120 first appeared as a state route around 1935. It was completely unpaved, but still a major route connecting three county seats: Buchanan, Dallas and Marietta. The original highway began at U.S. 27 in Buchanan extending east to U.S. 41 (now S.R. 5) in Marietta. In 1937, it was extended further east to Roswell to end at U.S. 19 (now S.R. 9). The earliest part to be paved was the portion through Dallas, which overlapped S.R. 6 (later U.S. 278, now S.R. 6 Business). While most of this route follows the highway of today, it appears that in Roswell, Coleman Road via Magnolia Street was the original route from near the Cobb County line to S.R. 9, but was relocated to its current route prior to 1945. This was only the beginning of many changes to come.

The original S.R. 120 was a continuous route, but it later had an eastern segment added from Lawrenceville to North Fulton County. When this eastern segment was designated in the early 1940's from Lawrenceville west to S.R. 141 (Medlock Bridge Road) in the Ocee community of what is now Johns Creek, it created a missing link situation between Roswell and S.R. 141. Also, it is noted that west of Duluth, this route was not the same as today. West of Duluth, this eastern leg of S.R. 120 joined S.R. 141 further south in the Warsaw community using what is today State Bridge Road, McClure Bridge Road and Lawrenceville Street, respectively, to where the route switches back today on Old Peachtree Road in downtown Duluth.

Note the missing link between Roswell and S.R. 141 in 1959. This is today the part of the highway that includes the interchange with S.R. 400 between Alpharetta and Duluth known as Old Milton Parkway, Kimball Bridge Road and Abbotts Bridge Road.

Interestingly enough, of all portions of the highway, the highway was actually paved first in Paulding County rather than in Cobb County. It was noted that this paving was completed by 1943. This newly paved road also apparently extended just over the Haralson County line to S.R. 113 in the Draketown community. In addition, the highway itself was also extended around that same time west of Buchanan to S.R. 100 in Tallapoosa near U.S. 78.

In Cobb County, paving was completed in 1945. It was also completed in Gwinnett during this time on the route east of Duluth, though the bridges on this part were built earlier. It should be noted that in Cobb County, significant realignment took place with at least three older alignments today named Old Dallas Road. The new route originally crested very steep hills prior to later four-laning.

By 1946, the portion west of Duluth of S.R. 120 was relocated to Abbotts Bridge Road in lieu of the more southern route, a move which delayed the paving of the route. However, it still ended at S.R. 141 like before. Across to the other side, the highway in Paulding and Haralson County had most of its bridges replaced with modern structures starting in 1946 continuing to 1949. Later in 1950, S.R. 120 was completely paved with exception to the route between Buchanan and Tallapoosa. This last part to be paved was finally finished in 1953.

The missing link on S.R. 120 in Johns Creek was interesting, because it proved to be only an 8-mile long gap when S.R. 120 was overlapped with a 6-mile stretch of U.S. 19 from Roswell to Alpharetta. Most likely, the road was not in condition previously, and it appears not to have been fully paved until the late 1950's. Basically, this gap took until 1961 to fill for a road that was otherwise designated in the 1940's. This is why today the roads in that area adhere more to their local names, and it is clear that three county roads were combined to create this route. These include parts of State Bridge Road (now Old Milton Parkway along S.R. 120), Kimball Bridge Road and Abbotts Bridge Road.

In the years after the highway was completed, Atlanta was growing toward the route. This demanded upgrades to the existing roadway. The first of such improvements included the widening of Roswell Road in Marietta in 1969 and the improvement of the interchange with S.R. 316 in 1979. However, the 1980's and 1990's were the most significant times of change for the highway. The first widening project along Roswell Road extended from S.R. 120 Loop east to Johnson Ferry Road in 1988 in addition to widening along the S.R. 9 overlap from Roswell to Alpharetta that same year. This was followed by widening of the portion from Johnson Ferry Road to S.R. 9 in Roswell in 1990.

Improvements continued through the 1990's. First, the steep intersection of S.R. 176 (Mars Hill Road) and S.R. 120 was leveled for additional lanes and better sight distance in 1992. This intersection is the location of the famed Lost Mountain Store, which still stands today in a neutered state as a bank. The intersection project was one of three in 1992. Also that year, S.R. 120 was relocated to a new four-lane by-pass south of Dallas as part of the reconstruction of U.S. 278. S.R. 360 was extended along part of this route to the new by-pass when it was completed. Lastly, the bridge over Pumpkinvine Creek was replaced west of Dallas.

In 1994, widening was completed along all of S.R. 120 from John Ward Road west to the Paulding County line, which unfortunately removed the steep hills that made the highway scenic before. In fact, prior to the four lane project, this road was known as "Lost Mountain Scenic Highway". The completion of this project was what began the suburbanization of West Cobb County and triggered the phenomenal sprawl in Paulding County today.

The last leg of the widening project completed was in 1997 in Paulding County extending from the Cobb County line to U.S. 278. This resulted in an upgrade of the already constructed two-lane connector from Old U.S. 278 (S.R. 6 Business) to modern U.S. 278. This was also the last major improvement to date along the western leg of the route aside from the Lane Creek bridge replacement in 2002.

The eastern leg, however, continued to see improvements until 2001, but only in locations around other road projects. In 1996, Riverside Parkway resulted in the widening of the route from S.R. 316 west to the new road project. In 1999, Sugarloaf Parkway, a county highway, included more widening near its intersection. The interchange with I-85, including widening of part of the road, was completed in 2001 with widening from Satellite Boulevard to Sugarloaf Parkway. Because these were tied to other road projects, a two-lane gap remains between Riverside and Sugarloaf Parkways.

Today, one part of S.R. 120 remains in limbo for a long needed widening project and that is Whitlock Avenue in Marietta. Note that S.R. 120 merges to two lanes between downtown Marietta and John Ward Road. The county hoped that Barrett Parkway would help relieve this bottleneck, but after many hearings to try to find alternatives to demolishing historic homes and businesses on Whitlock Avenue including using different streets and creating a one-way pair on Polk Street, no alternative has ever come to fruition. A six-laning of Roswell Road east of Marietta is also likewise not likely to get funded. This still does not compare, however, to the hopelessly congested two-lane roadway east of Alpharetta, which has been permanently stalled for any future widenings since 1995. Regardless, S.R. 120 has indeed been a highway of progress.