The following year he was elected House Speaker. After the Commonwealth of the Philippines was established 1935, Roxas became a member of the unicameral National Assembly, and served (1938–1941) as the Secretary of Finance in President Manuel L. Quezon's cabinet.
Pagkatapos maitatag ang Komonwelt ng Pilipinas (1935), naging kasapi si Roxas sa National Assembly, nagsilbi (1938-1941) bilang Kalihim ng Pananalapi sa gabinete ni Pangulong Manuel Quezon, at naihalal (1941) sa Senado ng Pilipinas.