The Peter Cornelius Archive is the largest and most important international collection of manuscripts by the Mainz composer and poet Peter Cornelius (1824–1874). The core of the collection consists of the estate acquired by the city of Mainz in 1950. The collection is still being systematically expanded through occasional purchases from antiquarian booksellers. It includes

• letters from and to Peter Cornelius as well as family correspondence
• Notebooks and diaries
• Music manuscripts in the form of autographs and transcriptions
• Music prints – often in first editions
• Poems by Peter Cornelius
• Pictures and various objects related to Peter Cornelius

The collection of letters, notebooks, poems, music manuscripts, and other individual items are listed in the Kalliope database, the union catalog for estates and autographs, where the remaining parts of the collection are also being gradually added. The music manuscripts are also indexed by the International Dictionary of Music (RISM).

The Scientific City Library has an outstanding collection of printed works from the Mainz-based publisher Jos[eph] Scholz. He was one of the first in Germany to focus his program primarily on children and young people. For over 100 years, he was known far beyond Germany's borders as a producer of picture sheets, games, paper theaters, and children's and young adult books.

The family business was originally founded in 1796 as a manufacturer of quill pens in Wiesbaden. From 1829 until World War II, it operated as a publishing house in Mainz. After the destruction of its headquarters in 1945, the publishing house was run from Wiesbaden until 1966, then returned to Mainz before being sold in 1970.

Since the 1970s, the Mainz City Library has been systematically building up its special collection Jos. Scholz special collection with the help of antiquarian acquisitions, including books (over 1,100 titles, including coloring books and scripts for paper theater plays), picture sheets (over 600, including over 400 paper theater sheets), and games (47 titles) from the former Mainz publishing house Jos. Scholz. All objects in this collection can be used in the reading room. The collection is fully cataloged in the online catalog.

The theater library consists of performance materials from the former Mainz City Theater (now the State Theater), which the City Library took over the storage and cataloging of in 1985. In 2002, ownership of the music collection was also retroactively transferred to the City Library. The collection is still available for loan to the State Theater; otherwise, the material can be viewed in the reading room of the City Library.

The sheet music, mainly from the 19th and 20th centuries and partly handwritten, covers the areas of opera, operetta, drama, and concert music. In addition to sheet music, the collection also includes librettos, prompt books, and theater programs. The Theater Library comprises

• approx. 600 scores and piano reductions
• approx. 100 librettos
• performance material (parts) for approximately 170 operas and 3,400 pieces of concert music.

It is fully listed in the online catalog of the city library.

One of the most significant bequests in the city library's collection is the Moyat Collection, which was left to the library in the will of Mainz merchant Jakob Moyat (1861-1933). This is a comprehensive collection of books comprising approximately 1,200 titles from all areas of ornithology, as well as nearly 200 letters. The book collection focuses on the 19th century, with a particularly large number of valuable illustrated table works. The collection also includes important ornithological bibliographies, encyclopedias, and specialist journals. The volumes in the Moyat Collection, all of which can be searched in the Mainz City Library's online catalog, can be used in the reading room.

The Mainz City Library holds part of the estate of Mainz-born composer Volker David Kirchner (1942–2020). The 33 handwritten scores date mainly from the years 2014–2019 and are recorded in the Kalliope database. Subject to copyright and exploitation rights, they are available for research, publication, and performance purposes.

The estate of composer and music lecturer Friedrich K. Wanek (1929–1991), who was born in Lugij (Romania) and worked in Mainz, includes music manuscripts, correspondence, audio recordings, concert programs and posters, newspaper reviews, sketchbooks, photographs, and poems. It is currently catalogued in the form of an inventory list and is available for use.

Born in Kaiserslautern, writer Susanne Faschon (1925–1995) is primarily known for her poems in dialect and standard German, but she also wrote narrative prose, radio plays, and texts for radio broadcasts. She worked as a librarian at Südwestfunk in Mainz, among other places. Her estate includes correspondence, poems, and other texts (in the form of typescripts, copies, and manuscripts), notebooks, newspaper articles, documents, photographs, and audio recordings. It has been cataloged in the form of an inventory list and is available for use.

In addition to the autographs from the collections and estates mentioned above, the City Library's autograph collection includes 30 letters from Beethoven to the music publisher Schott, 88 autographs from the Mainz Liedertafel collection, and around 75 other autographs (letters and musical manuscripts) from individuals connected with cultural life in Mainz (e.g., Wilhelm Heinse, Georg Forster, Carl Zuckmayer, and Hans Gál). The autograph holdings are currently being cataloged online in Kalliope. The Liedertafel autographs, the Beethoven letters, and the letters from the Moyat Collection have already been cataloged; further holdings will follow.