A number of A-26s were sold off as surplus after the end of the Second World War. As early as 1946, a few Invaders began to appear on US civilian rosters. These planes had come primarily from a batch of 28 A-26s than had gone directly to surplus as soon as they had left the factory. They were converted into executive transport aircraft by adding a few passenger seats along with some additional creature comforts and and some windows in the sealed-up bomb bay. A few corporations purchased some of these converted A-26s for use in ferrying their top executives around the country.
One of these civilian-registered Invaders was an A-26B which was sponsored by industrialist Milton Reynolds and flown by William P. Odon. It was named "Reynolds Bombshell" and was assigned the civilian registry NX67834. It twice broke Howard Hughes' prewar round-the-world speed record. Between April 12 and 16, 1947, it flew around the world in 78 hours 55 minutes 56 seconds. Less than four months later, Bill Odon broke his own record by again flying around the world, this time with an elapsed time of 73 hrs 5 mins 11 seconds, for an average flying speed of 310.6 mph.
Other surplus Invaders were acquired by civilian operators for use as aerial reconnaissance aircraft, geological surveyors, and aerial firefighters. Many had to be extensively modified for their new roles. Among these were the variants developed by the L. B. Smith company. The first of these was known as the Super 26. It was fitted with wingtip fuel tanks and had executive accommodations, but was otherwise similar to the basic Invader. Later conversions included the Smith Tempo series. The Smith Tempo I was powered by R-2800 B-series engines and had an unpressurized fuselage. The Smith Tempo II had R-2800 C-series engines and was equipped with full cabin pressurization. Both versions featured wingtip tanks and a new 9 foot 7.5 inch extension on the fuselage that could accommodate from ten to 13 passengers.
The Monarch 26 was a corporate aircraft conversion performed by the Rock Island Oil and Refining Co of Wichita, Kansas. The company purchased six surplus Invaders (ex-French AF aircraft that had served in Indochina) out of the Clark Field storage facility. The original Invader airframe was extensively reconfigured with re-contouring and extensive re-skinning. In order to provide for more interior cabin space, an new wing spar was designed to replace the original straight-through configuration and new upper and lower fuselage structures were extended from the rear spar to the fin root fillet. The new cabin could accommodate up to six passengers in a low-density arrangement, and featured floor heating, a restroom, catering facilities, and panoramic windows. There was an airstair door on the rear starboard side. The extended nose was 30 inches longer than the original. Because of a fear of wing failures, the Rock Island engineers decided not to add wingtip fuel tanks, but added additional fuel cells in the outer wing panels, which raised the total fuel capacity to 1012 US gallons. The cockpit was fitted with dual controls, new instrument and overhead panels, and "metalized" double-paned cockpit canopies.
Anticipating more orders, Rock Island acquired 30 more surplus Invader airframes, this time from storage at Davis Monthan AFB. However, the Monarch had only limited success in the corporate aircraft marketplace, with only 4 being completed, including three in-house. In addition, the Monarch found itself in competition with the first generation of corporate jets such as the Sabreliner, Jetstar, and Learjet. The majority of excess airframes that had been acquired were sold to parts brokers in 1969 but several examples went on to serve as aerial tankers in the USA and Canada. Some also ended up as flying warbirds, museum exhibits, and potential restoration projects.
Rock Island capitalized on its experience with Invader conversions by creating the Consort 26, which was designed for research and development purposes. All military equipment was removed, the bomb bay was sealed up, and a reinforced floor was added. Three Invader airframes were modified in this way and were sold or leased to aerospace companies as platforms for system development programs.
Perhaps the best-known of the civilian Invader conversions were those done by the On Mark Engineering Company of Van Nuys, California. Among these were the pressurized Marksman A, B, and C. The Marksman A had 2100 hp R-2800-83AM3 engines, the Marksman B had 2100 hp R-2800-83AM4A engines and wingtip fuel tanks, whereas the Marksman C had 2500 hp R-2800-CB-16/17s and internal auxiliary fuel tanks. On Mark provided the additional room for passengers in the cabin of the aircraft by removing the rear wing spar and substituting a circumferential ring bulkhead to which the wings were mounted in the same place as with the carry through spar. The forward wing spar was not changed because of the magnitude of the re-engineering that would have been required, which meant that crew members were left with the inconvenience of mounting the flight deck through a crawlway along the right side of the cabin beneath the spar.
All of the Marksmen had a redesigned and pressurized fuselage with a new flight deck, a DC-7-type heated windshield that was more resistant to bird strikes than the original B-26 windscreen, and improved brakes, deicing, soundproofing, radio/navigation and other systems. They could carry from 6 to 12 passengers. The base price of the Marksman was $257,430. Eight Marksman conversions have been identified (41-39221, 43-22416, 44-34415, 44-34526, 44-34567 44-34761, 44-35698, and 44-35870)
There was also an unpressurized version of the Marksman C known as the Marketeer that lacked the solid roof and the DC-6 cockpit glazing of the Marksman.
Several of the On Mark converted Invaders have survived into the 1990s.
AirSpray Ltd of Canada currently owns and operates 18 Invaders converted to the firefighting role. Each of them is equipped with a 1000-gallon tank for holding fire retardant.