His most notable work was developing comparative grammar studies of the Bantu languages, building on the pioneering work of Wilhelm Heinrich Immanuel Bleek. In his work, Meinhof looked at the common Bantu languages such as Swahili and Zulu to determine similarities and differences.
In his work, Meinhof looked at noun classes with all Bantu languages having at least 10 classes and with 22 classes of nouns existing throughout the Bantu languages, though his definition of noun class differs slightly from the accepted one. While no language has all 22, Sesotho has 18. All Bantu languages have a noun class specifically for humans (sometimes including other animate beings).
Meinhof also looked at other African languages including the Kordofanian languages, Bushman, Khoikhoi and Hamitic languages. During his career, Meinhof published several books on African languages and made recordings of East African music in 1902 which was one of the first recordings made of traditional African music.