In 1947, a group of civic leaders held a national competition to select a design for the main portion of the Memorial space. Eero Saarinen won this competition with plans for a 590-foot (180-metre) catenary arch to be placed on the banks of the Mississippi River. However, these plans were modified over the next 15 years, placing the arch on higher ground and adding 40 feet (12 m) in height and width.

Saarinen developed the shape with the help of architectural engineer Hannskarl Bandel. It is not a pure inverted catenary. Saarinen preferred a shape that was slightly elongated and thinner towards the top, a shape that produces a subtle soaring effect, and transfers more of the structure's weight downward rather than outward at the base.

When Saarinen won the competition, the official notification went to his father, architect Eliel Saarinen, who had also submitted an entry. The family celebrated with a bottle of champagne, and two hours later an embarrassed official called to say the winner was, in fact, the younger Saarinen. The elder Saarinen then broke out a second bottle of champagne to celebrate his son's success.

The construction of the Arch began February 12, 1963 and was completed on October 28, 1965, costing less than US$15 million to build. Along with all other historical areas of the National Park Service, the memorial was listed on the National Register of Historic Places on October 15, 1966. Vice President Hubert Humphrey and Secretary of the Interior Stewart Udall dedicated the Arch on May 25, 1968.

In 1984, Congress authorized the enlargement of the Memorial to include up to 100 acres on the east bank of the Mississippi River in East St. Louis, Illinois. Funds were authorized to begin land acquisition, but Congress placed a moratorium upon NPS land acquisitions in fiscal year 1998. The moratorium continued into the 21st century, with expansion becoming less likely because of the construction of a riverboat gaming facility and related amenities.

The Arch, designed by Finnish architect Eero Saarinen, stands 630 feet (192 m) tall, and is 630 feet (192 m) at its widest point. The cross-sections of its legs are equilateral triangles, narrowing from 54 feet (16.5 m) at the base to 17 feet (5.2 m) at the top. Each wall consists of a stainless steel skin covering reinforced concrete from ground level to 300 feet (91 m) or carbon steel and rebar from 300 feet (91 m) to the peak.

The interior of the Arch is hollow and contains a unique transport system leading to an observation deck at the top. The interior of the Arch also contains two emergency stairwells in the event of a need to evacuate the Arch or if a problem develops with the tram system.

Underneath the Arch is a visitor center, entered from a descending outdoor ramp starting at either base. Within the center is the Museum of Westward Expansion, exhibits on the history of the St. Louis riverfront, and tram loading and unloading areas. Tucker Theater, finished in 1968 and renovated 30 years later, has about 285 seats and shows a documentary (Monument to the Dream) on the Arch's construction.

Odyssey Theater was completed in 1993 and has 255 seats. It was the first 70 mm film theater to be located within on NPS grounds and operated by the NPS. It runs films from a rotating play list.

Visitors pass through security checkpoints at each entrance to the Arch, before being allowed access to the visitor center. Security was increased as result of a 1997 Congressional mandate to establish a Counter-Terrorism Program at the park. The NPS used the increased funding to purchase magnetometers and x-ray equipment for visitor screening and 25 CCTV cameras scattered throughout the grounds of the memorial.

When the Arch was first built, the only way to the top was to climb 1000-plus stairs. By 1968 however a unique tram system designed by Richard Bowser had been installed.

From the visitor center one may move to either base of the Arch and enter the tramway much as one would enter an ordinary elevator, through narrow double doors. Passing through the doors the passengers in groups of five enter a horizontal cylindrical compartment containing five seats and a flat floor. Because of the car shape, the compartments have sloped ceilings which are low enough to force taller riders to lean forward while seated (for this reason it's recommended that the tallest of the five passengers in the car sit in the center seat facing the door). Several of these compartments are linked to form a train. These compartments each individually retain an appropriate level by rotating, which allows them to maintain the correct orientation while the entire train follows curved tracks up one leg of the arch.

There are two groups of these vertical tramways, one on the north end and the other on the south end of the Arch. The north queue area includes displays which interpret the design and construction of the Gateway Arch; the south queue area includes displays about the St. Louis riverfront during the mid-19th century.

Each tram is made up of eight cars. The trip to the top of the Arch takes four minutes, and the trip back down takes three minutes. The car doors have narrow glass panes, allowing passengers to see the interior of the Arch during the trip.

The tram is operated by the quasi-governmental Bi-State Development Agency under an agreement with the NPS.

Near the top of the arch, a rider will exit the compartment and climb a slight grade to enter the arched observation area. Small windows, almost invisible from the ground, allow views across the Mississippi River and southern Illinois with its prominent Mississippian culture mounds to the east at Cahokia, and the city of Saint Louis and the Great Plains to the west. On a clear day, one can see up to 30 miles.(48 km)