In 1970 Marcos embarked on totally new concept, - the 4-seater Marcos. It was code named the M70 and marketed as the Marcos Mantis 2 + 2.

The car was styled by Dennis Adams with input from Jem Marsh. Brian Cunnington (Marcos's ex-Lotus design engineer) designed the chassis and productionised the vehicle. 

The Mantis was powered by the 2.5 PI straight six Triumph engine coupled to the Triumph 4 speed gearbox with overdrive. The suspension was similar to the 3 Litre but featured trailing links and an A-frame to locate the Ford Capri rear axle. The futuristic design was greeted with mixed feeling and certainly grabbed attention at the Earls Court Motor Show in 1971.

The Mantis was probably launched at the wrong time for Marcos as it coincided with the collapse of the American market. With the move to the new factory in Westbury, and the development costs of the car, only 32 cars were ever made. This being said, all 32 cars sold within an eleven-month period prior to the factory closing in 1972.