At the height of the Cold War, Martin Mercators flew clandestine ferret missions to gather electronic intelligence from China, North Korea, the USSR and Vietnam. The Mercator flew with just one US Navy patrol squadron (VP-21) as the P4M-1 before entering the sinister world of Cold War 'black cover' reconnaissance as the P4M-1Q. Officially the P4M-1Q Mercator belonged first to 'warning' and later to 'electronic countermeasures' squadrons even though, in fact, it never performed either duty.

Contemporary with the Lockheed P2V Neptune, The Mercator was conceived in 1944 as a replacement for the Consolidated PB4Y Privateer, and particularly for the specific role of mine-laying during the attack on, and invasion of Japan. The Neptune, on the other hand, was always intended as a replacement for the smaller, shorter-ranged, twin-engined Lockheed Ventura, and had been launched (as a private venture) in September 1941. This "head start" was destined to have far-reaching effects. The Martin Model 219 reflected the Navy's interest in combining the benefit of high performance over target with very long range. The solution, in the case of the Mercator, was a 'mix' of reciprocating and jet engines. The Mercator's power plant comprised two 2,975hp Pratt & Whitney R-4360 Wasp Major radial engines (among the biggest and most complex piston engines ever put into the air) plus two 3,8251b thrust Allison J33-A-17 turbojet engines, with each nacelle holding one reciprocating and one jet engine. This compound prop-jet arrangement has been compared with the Neptune, but the latter did not have jets added to its two piston engines until long after entering service, and then only as an afterthought. The Mercator was a prop-jet warplane from the beginning.

Unfortunately for Martin, Lockheed's Ventura replacement emerged as a much more capable aircraft than the Navy had asked for. About one-third smaller than the Mercator, the Neptune was powered by Wright Cyclone engines with roughly the same output as the Mercator's primary reciprocating engines, yet was much lighter. It was able to carry a similar fuel load. As a result, the Neptune had a longer range than the four-engined Mercator, and had a significantly lower price tag. The Neptune was also ready first and this would eventually prove to be critical. Martin would produce only 19 Mercators for the Navy.

Most of the P4M-1s were delivered to VP-21, stationed at Patuxent River Naval Air Station in Maryland and later at Port Lyautey in Morocco. Fast and heavily armed, the Mercators were better suited to bombing and mine-laying missions in hostile airspace than to antisubmarine patrol. Maintenance problems with the turbojets led to temporary restrictions on their use, but after a series of successful and well publicized long-range flights, the restrictions were lifted and squadron morale improved. Yet however successful the P4M-1 was in service, it made little sense to have a single patrol squadron operating Mercators, while the rest of the patrol community operated Neptunes.

While no one in the higher echelons of the patrol community really knew what to do with the Mercator, the lumbering Privateer was beginning to show increasing vulnerability in the electronic and radar reconnaissance roles. The Mercators had obvious potential as PB4Y replacements. Diverting Neptunes as Privateer replacements made no sense at all, since they were too small, too slow and too limited even before the addition of heavy electronic equipment and extra crew stations. Beginning in 1950, all but one of the production models (121452, lost in an accident in Chesapeake Bay on March 8, 1951) were specifically modified by the Navy for electronic reconnaissance as P4M-lQs, the last conversion being completed in October 1951. Externally, the electronics aircraft could be recognized by their four white ventral radomes, rather than the two of the original production version. Several of the Q ships also had large dorsal radomes in place of the upper gun turrets. The crew was increased to 14 (adding five more electronics operators and an aircraft captain, who could also serve as a relief gunner), loaded weight increased to 88,378 pounds, although the range dropped to 2,000 miles.

P4M-1Q crews trained with composite squadron VC-11 at NAS Miramar, California during April-September 1951, and four aircraft began operations from Naval Station Sangley Point, in the Philippine Islands in October. Serving with a variety of small, secret units, their tail numbers sometimes disguised, Mercators of the "Special Products Division" of the Air Operations Department and "Detachment A" of Airborne Early Warning Squadrons One and Three (VW-1 and VW-3) were dispatched to monitor radar and radio signals along the coasts of the Soviet Union and its allies, Communist Vietnam, China and North Korea. Extended patrol missions, beginning in the Philippines and ending in Japan, often lasted as long as 14 hours. They were conducted mainly at night, the planes loaded with electronic black boxes and the large crew of pilots, operators and gunners.

Occasionally these spy flights met with opposition. After a Moraccan-based Mercator crashed in the eastern Mediterranean in February 1952, rumors suggested that it had been attacked. Another Mercator was definitely shot down by Chinese MiG-15 fighters near Shanghai in 1956. Still another fought off attacks by two North Korean MiG-17s in 1959, the tail gunner being wounded. Back in the Mediterranean, a P4M-1Q of VQ-2, operating from Incirlik, Turkey, crashed with the loss of all sixteen crewman aboard in January, 1960. The last Mercator was retired in July 1960.

Perhaps minor in scale compared to the widespread service of the Lockheed Neptune, the unusual and very important surveillance role carried out by the Mercator required a highly specialized airplane with great endurance, substantial fuel and electronics capacity, effective defensive armament, and sudden dash speed. The Martin P4M-1 Mercator met all these needs handsomely during a crucial period in American history. Today, 40 years later, not a single example survives.