Mehanna travelled to the United Kingdom in 1937 on a military scholarship, then returned a year later in order to work as a teacher at the Artillery School.

After the Revolution took place in July 1952, Mehanna was appointed Minister of Transport in the first post-revolutionary government, which was headed by Ali Maher.

Mehanna was chosen by Nasser to serve as a representative of the Army on the three-member Regency Body established during the nominal reign of infant king-in-exile Fuad II.

However, the Regency Body had no real powers, these having been assumed by the Revolutionary Command Council (RCC).

Historians of this period agree that one of the main reasons for the hostility that arose between Mehanna and the RCC was that the former actually took his title seriously while Nasser and his colleagues had no intention whatsoever of allowing him to exercise the powers that came with it.