He was the leading scholar of the composer Giovanni Pierluigi da Palestrina, about whose life and music he wrote numerous studies.
Jeppesen demonstrated early musical talent at age 10 when he was first encouraged by Hakon Andersen and Paul Hellmuth, although he was largely self-taught.
[2] Following his retirement in 1957, Jeppesen resided in Italy, enabling him to make several discoveries in Italian libraries culminating in his magnum opus, La frottola (1968–70), a detailed study and bibliography of frottole, the leading genre of Italian popular, secular songs in the late fifteenth and early sixteenth century.
His doctoral thesis was expanded in 1923 and appeared in English in 1927 as The Style of Palestrina and the Dissonance – a work which The New Grove cites as being the "most distinguished and influential example of stylistic analysis at this time".
Jeppesen engaged in much primary research – locating early manuscript and print copies of old scores and preparing editions with annotations and commentary.