Developed by Jet Propulsion Laboratory, and originally planned to be a purpose-built probe launched summer 1962, Mariner 1's design was changed when the Centaur proved unavailable at that early date.

Mariner 1 carried a suite of experiments to determine the temperature of Venus as well to measure magnetic fields and charged particles near the planet and in interplanetary space.

With the advent of the Cold War, the two then-superpowers, the United States and the Soviet Union, both initiated ambitious space programs with the intent of demonstrating military, technological, and political dominance.

A plan drafted January 1959 involved two spacecraft evolved from the first Pioneer probes, one to be launched via Thor-Able rocket, the other via the yet-untested Atlas-Able.

The Thor-Able probe was repurposed as the deep space explorer Pioneer 5, which was launched March 11, 1960, and designed to maintain communications with Earth up to a distance of 20,000,000 mi (32,000,000 km) as it traveled toward the orbit of Venus.

The main body of the craft was hexagonal with six separate cases of electronic and electromechanical equipment: At the rear of the spacecraft, a monopropellant (anhydrous hydrazine) 225 N[11] rocket motor was mounted for course corrections.

[3]: 175–176 Temperature control was both passive, involving insulated, and highly reflective components; and active, incorporating louvers to protect the case carrying the onboard computer.

[10]: 331  An on-board magnetometer and suite of charged particle detectors could determine if Venus possessed an appreciable magnetic field and an analog to Earth's Van Allen Belts.

[15] As the Mariner spacecraft would spend most of its journey to Venus in interplanetary space, the mission also offered an opportunity for long-term measurement of the solar wind of charged particles and to map the variations in the Sun's magnetosphere.

With payload space at a premium, project scientists considered a camera an unneeded luxury, unable to return useful scientific results.

Carl Sagan, one of the Mariner R scientists, unsuccessfully fought for their inclusion, noting that not only might there be breaks in Venus' cloud layer, but "that cameras could also answer questions that we were way too dumb to even pose".

[16] The launch window for Mariner, constrained both by the orbital relationship of Earth and Venus and the limitations of the Atlas Agena, was determined to fall in the 51 day period from July 22 through September 10.

[3]: 174  The Mariner flight plan was such that the two operational spacecraft would be launched toward Venus in a 30-day period within this window, taking slightly differing paths such that they both arrived at the target planet within nine days of each other, between December 8 and 16.

Deep space support was provided by three tracking and communications stations at Goldstone, California, Woomera, Australia, and Johannesburg, South Africa, each separated on the globe by around 120° for continuous coverage.

Its flight trajectory began to point downward and to the left of where it was supposed to be, creating the danger that it could impact in the crowded Atlantic shipping lanes.

[10]: 87 Because of the gradual rather than sharp deviation from its course, JPL engineers suspected the fault lay in the flight equations loaded into the computer that guided Atlas-Agena from the ground during its ascent.

[17] After five days of post-flight analysis, JPL engineers determined what had caused the malfunction on Mariner 1: an error in the guidance computer logic combined with a hardware failure.

It was an adaption of the Mod III guidance system used on Atlas B, C, and D missiles which had the original vacuum tube electronics converted to transistors, but the modification had been done hastily and was unreliable.

The procedures implemented as a result served NASA well, ultimately salvaging the Project Apollo Moon landings; though there were program errors in the Lunar Excursion Module software during descent, they did not cause mission failure.