Controversy exists regarding the involvement of the primary motor cortex (M1) during motor imagery (MI) and also regarding the differential somatotopic representation of motor execution (ME) and mental simulation of movement, that is, MI within M1. Although some research reported clear M1 involvement during MI without overt motor output, others did not. However, possible somatotopic representation between execution and imagery has not been clearly investigated to date. The aim of the present study was to aid in the resolution of this controversy by investigating the possible involvement of M1 during MI, and the differential, within M1, somatotopic representation between execution and imagery by quantitatively assessing different location markers such as activation peak and center of mass as well as intensity differences between the two tasks in case of with and without the overlap between the two representations. Forty-one healthy volunteers participated in two functional MRI runs of mouth-stretching ME and MI tasks. Our findings suggest the clear involvement of M1 (BA 4) during MI with lower signal intensity compared with ME, and further showed distinct centers for each representation along the y-axis (anteroposterior plane), with MI showing more involvement of the anterior sector of M1 (BA 4a), whereas ME recruited more of the posterior sector (BA 4p). These results parallel the pioneering findings of a functional distinction between BA 4a and BA 4p, where BA 4a is more involved in the cognitive aspects of MI, whereas BA 4p is more related to executive function, promoting the idea of distinctive somatotopic mapping between execution and imagery within M1 sectors.