Construction of the Piers
A thick layer of glacier marl over the rock, the depth of the river and the speed of the current obliged the engineers to bypass standard techniques and devise a method to erect the bridge piers that was better suited to these conditions.

Because of the depth of the St. Lawrence, the engineers refused to use cofferdams, and instead chose pneumatic caissons, a construction method that had not been used in Montreal in 25 years.

The steel walls of the caissons were built in separate parts by Dominion Bridge at its Lachine works. They were then transported by barges from Ville LaSalle to Dufresne Engineering's yard located downstream of the bridge, for assembly.

Rings were welded to the outside walls to insert heavy steel piles 10 inches (0.254 m) in diameter. After they were assembled, the caissons were attached to two beds mounted on barges solidly attached to one another and then transported to the exact spot where the piers were to be built. The workers drove the piles into the rings to ensure the stability of the caissons while they were being sunk.

A batching plant that would supply the concrete for the caissons was also installed on a barge. Another was used for the compressed air plant. The working chambers were connected with the outside through three shafts with airlocks. The workers used the middle shaft, while excavated materials were removed through those on either end. The excavation work was fairly problem-free. Blasting was even done to dislodge large chunks of rock. 

However, there were problems installing the two caissons. One of them was to be erected less than 12 inches (0.305 m) from the corresponding pier of the old bridge, which forced the engineers to give it a special shape. Its dimensions (32 feet by 71 feet 11 inches (9.75 m by 21.94 m)) made it too heavy for Dufresne Engineering's shoring, so they had to have Dominion Bridge assemble it in the Lachine Canal and float it to the bridge site.

Because of its considerable size, special shape and the strong current, a barge had to be added at the upstream end of the caisson. The barge was attached to the caisson with cables and a baffler was added to reduce its resistance to the swift currents. At the upstream Canadian Pacific Bridge, in a 12-foot (3.66-m) per second current, anchors were dropped from the barge on the upstream side. Powerful winches on the immobilized barge held the caisson for the last 599 feet, 6 inches (182.80 m) of the trip. Once it arrived at its destination, a third cable was affixed to an attachment point on the South Shore to bring the caisson into final position.

An adventure awaited the other caisson, which went adrift. The vibrations caused by the strong currents broke the temporary anchoring cables and the concrete-filled caisson grounded on a bank 999 feet, 3 inches (304.67 m) downstream from the bridge. Fortunately, the caisson was recovered, despite the strong current.

Raising of the Bridge Sections Straddling the Seaway
In 1958-1959, the St. Lawrence Seaway Authority raised the sections of the bridge straddling the Seaway, which was under construction at the time. The spans on the south side, beginning at pier 14, were jacked up to provide clear passage for ships plying the Seaway. As a result, the raised part of the bridge, representing half of the bridge's length, is under federal jurisdiction.

Construction of a Second Bridge
In August 1963, it was felt that the Mercier Bridge's two lanes were inadequate for the constantly increasing traffic travelling across it, so the Quebec Department of Public Works had a second bridge built downstream of the first. At the same time, the deck of the old bridge was modified to match its capacity to that of its "twin." Minor changes were also made to the piers the entire surface of each pier was covered with a new layer of reinforced concrete and their shape was changed to match the piers of the new bridge. The "new" bridge spans the St. Lawrence River and Seaway along the west side of the original.

All that remains of the 1932 bridge is the section between both shores, that is, the concrete infrastructure and steel superstructure.

By twinning the old and new bridges, the number of lanes between the two shores was increased from two to four. Southbound traffic travels over the old bridge while northbound traffic crosses the new one.