After he had attended Latin school at Königsberg (1737–1741), his parents wanted him to enter the church, but after two years at the University of Wittenberg he determined to live the life of a scholar.

To them he owed a thorough grounding in ancient history and Roman antiquities and literature; and from them he learned a pure and vivid Latin style.

[1] Ruhnken followed the advice of his friends at Wittenberg and early in 1744 went to the University of Leiden, where, stimulated by the influence of Richard Bentley, Tiberius Hemsterhuis had founded the only real school of Greek learning on the Continent since the days of Joseph Justus Scaliger and Isaac Casaubon.

[1] As Ruhnken's reputation spread, many efforts were made to attract him back to Germany, but after settling in Leiden, he only left the country once, when he spent a year in Paris, ransacking the public libraries (1755).

Ruhnken's later years were clouded by severe domestic misfortune, and by the political commotions which, after the outbreak of the war with England in 1780, troubled the Netherlands without ceasing, and threatened to extinguish the University of Leiden.

In his bearing towards other scholars Ruhnken was generous and dignified, distributing literary aid with a free hand, and meeting onslaughts for the most part with a smile.

In the records of learning he occupies an important position, as a principal link in the chain which connects Bentley with the modern scholarship of the European Continent.