The body of his works is considerable, the earliest known being a group of five anonymous poems included in the "Songs of Divers Noblemen and Gentlemen," appended to Newman's edition of Sir Philip Sidney's Astrophel and Stella, which appeared in 1591. In 1595, Poemata, a collection of Latin panegyrics, elegies and epigrams was published, winning him a considerable reputation. This was followed, in 1601, by a songbook, A Booke of Ayres, with words by himself and music composed by himself and Philip Rosseter. The following year he published his Observations in the Art of English Poesie, "against the vulgar and unartificial custom of riming," in favour of rhymeless verse on the model of classical quantitative verse. Campion's theories on poetry were demolished by Samuel Daniel in "Defence of Rhyme" (1603).
In 1607, he wrote and published a masque for the occasion of the marriage of Lord Hayes, and, in 1613, issued a volume of Songs of Mourning: Bewailing the Untimely Death of Prince Henry, set to music by John Cooper (also known as Coperario). The same year he wrote and arranged three masques: The Lords' Masque for the marriage of Princess Elizabeth; an entertainment for the amusement of Queen Anne at Caversham House; and a third for the marriage of the Earl of Somerset to the infamous Frances Howard, Countess of Essex. If, moreover, as appears quite likely, his Two Bookes of Ayres (both words and music written by himself) belongs also to this year, it was indeed his annus mirabilis.
In 1615, he published a book on counterpoint, A New Way of Making Fowre Parts in Counterpoint By a Most Familiar and Infallible Rule, a technical treatise which was for many years the standard textbook on the subject. It was included, with annotations by Christopher Sympson, in Playford's Brief Introduction to the Skill of Musick, and two editions appear to have been published by 1660.
Some time in or after 1617 appeared his Third and Fourth Booke of Ayres. In 1618 appeared the airs that were sung and played at Brougham Castle on the occasion of the King's entertainment there, the music by George Mason and John Earsden, while the words were almost certainly by Campion. In 1619, he published his Epigrammatum Libri II. Umbra Elegiarum liber unus, a reprint of his 1595 collection with considerable omissions, additions (in the form of another book of epigrams) and corrections.