Following his meeting with Hasan Al Banna Ashmawi joined the Muslim Brotherhood in 1937.[1] Next year Ashmawi was appointed editor-in-chief of Al Nadhir, a weekly journal started by the Brotherhood.[1][2] However, he left the Brotherhood after internal disputes stopping the publication of the journal and involved in the establishment of another Islamic group entitled the Society of Mohammad's Youth.[1]
Later he restored his relations with Al Banna and became the head of secret group within the Brotherhood.[1] Under his leadership the secret apparatus gained considerable autonomy to the extent that Al Banna had no power over it.[3] Ashmawi was part of the extremist faction and was appointed the deputy secretary of Al Banna in 1947, replacing Ahmad Mohammad Al Sukkari in the post.[3] Abdul Rahman Al Sanadi, on the other hand, succeeded Ashmawi as the head of the Brotherhood's secret network.[1]
In 1949 Al Banna was assassinated, and Ashmawi and Hasan Ismail Al Hudaybi became the leaders of the Brotherhood which was banned in 1948.[4] In 1951 Ashmawi launched an Islamic journal, Al Dawa, which was the official organ of the group.[5] In 1953 he and Mohammad Al Ghazali, another senior Brotherhood figure, were dismissed from the group due to their conflict with Hasan Ismail Al Hudaybi.[6] Both Ashmawi and Al Ghazali were also arrested and jailed as part of Gamal Abdel Nasser's crackdown against the Brotherhood.[6]