He studied philology at Bonn, obtaining his degree in 1843 with a thesis titled Quaestiones Terentianae. From 1847 to 1849 he was a teacher in Elberfeld, afterwards moving to England, where he taught school in Liverpool until 1863. He returned to Germany as a lecturer at the University of Heidelberg, where in 1873 he was appointed professor. He died in Heidelberg.
Ihne is remembered for the classic Römische Geschichte (History of Rome), a work published in eight volumes from 1868 to 1890, and also translated into English. Other works on Roman history by Ihne include:
Forschungen auf dem Gebiet der rom Verfassungsgeschichte (Research in the field of Roman Constitutional history), 1847.
Zur Ehrenrettung des Kaisers Tiberius (A plea for the Emperor Tiberius), 1892.
History of Rome in English (at Internet Archive): five volumes from 1871 onwards: Volume 1 from Aeneas to the Conquest of Italy; Volume 2 covering the Punic Wars; Volume 3 covering from 200 BC to 133 BC; Volume 4 covering Roman institutions and the Gracchi; Volume 5 from the Jugurthine War to Sulla.
Römische Geschichte in German (also at Internet Archive): remaining three volumes of Roman History covering roughly the period from Sulla's death to the dominance of Octavian / Augustus following the Battle of Actium in 31 BC: Volume 6 and Volumes 7 and 8 combined