The members of the New Mexico class received a series of modifications even while under construction, including the already noted alterations to the secondary battery, which was authorized on 7 February 1918 by which time New Mexico had been completed with her original twenty-two guns.
The other two ships had already had their casemates built into their hulls, so they were simply plated over, unlike the later Tennessees, which had their hulls faired in.
Their conning towers were modified with a newly developed bridge arrangement that became standard for all American battleships of the period.
It consisted of a fully enclosed navigation bridge on the forward side of the tower, with a large chart house located behind the bridge.
Atop the structure was an enclosed torpedo defense station.
Starting in 1921, the Navy began installing aircraft catapults on its battleships to allow them to operate floatplanes for aerial reconnaissance and fire observation, and the three New Mexicos were among the vessels to receive a Mark II catapult.
The ships initially carried Hanriot HD.2 and then Vought UO-1s in the 1920s, which were replaced with  Vought O2U Corsairs toward the end of the decade.
These, and improved O3U variants served aboard the ships  until 1938, when these were replaced with Curtiss SOC Seagulls.
Beginning in August 1940, the ships began to receive Vought OS2U Kingfishers.

The three ships were heavily modernized in the early 1930s.
Their received an additional 2 in of special treatment steel for their main deck armor, bringing the total thickness to 5.5 in (140 mm).
A second torpedo bulkhead was installed outboard of the original one, and all three ships received anti-torpedo bulges to further enhance their resistance to underwater attack.
Both lattice masts were removed; a heavy tower bridge was built in place of the fore mast, and a light pole mast was erected in place of the main mast.
During the installation of her new bridge, Idaho was fitted out as a flagship, which included the addition of a flag bridge for the admiral and his staff.
The ships' armament was also revised, with the main battery turrets being modified to allow elevation to 30 degrees, greatly extending the range of the guns.
Two of the 5-inch guns were removed, and eight 5-inch/25 caliber anti-aircraft guns were installed.
These alterations greatly increased their displacement, to 33,420 long tons (33,960 t) standard and 36,157 long tons (36,737 t) full load.
All three ships had their propulsion systems overhauled, receiving new Westinghouse turbines and six BuEng express boilers.
Despite the increases in displacement resulting from the additional armor plating and the bulges, the new powerplant increased speed to 22 knots (41 km/h; 25 mph) from 40,000 shaft horsepower (30,000 kW).
The ships' crews increased significantly, to 1,443.

Immediately after the Japanese attack on Pearl Harbor on 7 December 1941 (while the New Mexico-class ships were in the Atlantic), the ships were docked for improvements to their anti-aircraft armament, receiving a battery of 1.1 in (28 mm) anti-aircraft guns.
The ships also had Mark 3 fire-control radars installed.
The Navy considered removing their heavily armored conning towers to compensate for the increased weight of the new equipment, but the work could not be accomplished in the eight-week overhaul period that had been scheduled, so they retained their original towers, unlike many of the other older battleships that were rebuilt early in the war.
Since many of the American battleships had been sunk or damaged at Pearl Harbor, the three New Mexicos were too valuable to be removed from operational status for the necessary amount of time to substantially rebuild them.
As a result of these limitations, New Mexico and Mississippi received a series of rolling changes over the course of late 1942 and into 1943; while at Pearl Harbor in October 1942, they had four of their 5-inch /51 guns removed, along with their directors, to free up space for a pair of quadruple Bofors 40 mm (1.6 in) guns and the shielded tubs for the 1
The members of the New Mexico class received a series of modifications even while under construction, including the already noted alterations to the secondary battery, which was authorized on 7 February 1918 by which time New Mexico had been completed with her original twenty-two guns. The other two ships had already had their casemates built into their hulls, so they were simply plated over, unlike the later Tennessees, which had their hulls faired in. Their conning towers were modified with a newly developed bridge arrangement that became standard for all American battleships of the period. It consisted of a fully enclosed navigation bridge on the forward side of the tower, with a large chart house located behind the bridge. Atop the structure was an enclosed torpedo defense station. Starting in 1921, the Navy began installing aircraft catapults on its battleships to allow them to operate floatplanes for aerial reconnaissance and fire observation, and the three New Mexicos were among the vessels to receive a Mark II catapult. The ships initially carried Hanriot HD.2 and then Vought UO-1s in the 1920s, which were replaced with  Vought O2U Corsairs toward the end of the decade. These, and improved O3U variants served aboard the ships  until 1938, when these were replaced with Curtiss SOC Seagulls. Beginning in August 1940, the ships began to receive Vought OS2U Kingfishers.
The three ships were heavily modernized in the early 1930s. Their received an additional 2 in of special treatment steel for their main deck armor, bringing the total thickness to 5.5 in (140 mm). A second torpedo bulkhead was installed outboard of the original one, and all three ships received anti-torpedo bulges to further enhance their resistance to underwater attack. Both lattice masts were removed; a heavy tower bridge was built in place of the fore mast, and a light pole mast was erected in place of the main mast. During the installation of her new bridge, Idaho was fitted out as a flagship, which included the addition of a flag bridge for the admiral and his staff. The ships' armament was also revised, with the main battery turrets being modified to allow elevation to 30 degrees, greatly extending the range of the guns. Two of the 5-inch guns were removed, and eight 5-inch/25 caliber anti-aircraft guns were installed. These alterations greatly increased their displacement, to 33,420 long tons (33,960 t) standard and 36,157 long tons (36,737 t) full load. All three ships had their propulsion systems overhauled, receiving new Westinghouse turbines and six BuEng express boilers. Despite the increases in displacement resulting from the additional armor plating and the bulges, the new powerplant increased speed to 22 knots (41 km/h; 25 mph) from 40,000 shaft horsepower (30,000 kW). The ships' crews increased significantly, to 1,443.
Immediately after the Japanese attack on Pearl Harbor on 7 December 1941 (while the New Mexico-class ships were in the Atlantic), the ships were docked for improvements to their anti-aircraft armament, receiving a battery of 1.1 in (28 mm) anti-aircraft guns. The ships also had Mark 3 fire-control radars installed. The Navy considered removing their heavily armored conning towers to compensate for the increased weight of the new equipment, but the work could not be accomplished in the eight-week overhaul period that had been scheduled, so they retained their original towers, unlike many of the other older battleships that were rebuilt early in the war. Since many of the American battleships had been sunk or damaged at Pearl Harbor, the three New Mexicos were too valuable to be removed from operational status for the necessary amount of time to substantially rebuild them. As a result of these limitations, New Mexico and Mississippi received a series of rolling changes over the course of late 1942 and into 1943; while at Pearl Harbor in October 1942, they had four of their 5-inch /51 guns removed, along with their directors, to free up space for a pair of quadruple Bofors 40 mm (1.6 in) guns and the shielded tubs for the 1
New Mexico-class battleship