Mohl's early investigations on the structure of palms, of cycads, and of tree ferns permanently laid the foundation of all later knowledge of this subject: so also his work on Isoetes (1840). His later anatomical work was chiefly on the stems of dicotyledons and gymnosperms; in his observations on cork and bark he first explained the formation and origin of different types of bark, and corrected errors relating to lenticels. Following on his early demonstration of the origin of stomata (1838), he wrote a classical paper on their opening and closing (1850).
In 1843 he started the weekly Botanische Zeitung in conjunction with Schlechtendal, which he edited jointly till his death. He was never a great writer of comprehensive works; no text-book exists in his name, and it would indeed appear from his withdrawal from co-operation in Hofmeister's Handbuch that he had a distaste for such efforts. In 1850, he was elected a foreign member of the Royal Swedish Academy of Sciences. In his latter years his productive activity fell off, doubtless through failing health, and he died suddenly at Tübingen on 1 April 1872.