In 1890, he joined the faculty of the University of Athens and taught history and ancient literature. He became Provost of the University in 1893, serving in that capacity twice, 1893–1894 and 1912–1913.
After 1903, Lambros started an academic movement called Neos Hellenomnemon (Νέος Ἑλληνομνήμων) which studied the scientific and philosophical developments of the Greek-speaking world during the Byzantine and Ottoman eras.
In October 1916 with Greece in the midst of the National Schism and under two governments (Eleftherios Venizelos in Thessaloniki and King Constantine in Athens), the former Liberal and associate of Venizelos accepted the King's commission to form a government in Athens. Eventually, riots took place in Athens (the Noemvriana), for which Lambros was judged responsible due to mis-management. He resigned as Prime Minister. After the exile of the king in summer 1917, Lambros was put in internal exile by the Venizelists, in Hydra and Skopelos.
He died in Skopelos on 23 July 1919.
- Jovanovski, Dalibor; Minov, Nikola (2017). "Ioannis Kolettis. The Vlach from the ruling elite of Greece". Balcanica Posnaniensia. Adam Mickiewicz University in Poznań. 24 (1): 222–223. ISSN 2450-3177. Retrieved 8 May 2017.
- Note: Greece officially adopted the Gregorian calendar on 16 February 1923 (which became 1 March). All dates prior to that, unless specifically denoted, are Old Style.