The generally accepted first computer scorewriter was SCORE, written by Leland Smith of Stanford University. Wikipedia dates its early development to 1967.
Composer Robert Morris published a short, engaging article on the history of music copying in his and his contemporaries lifetimes.1 In it, he traces the evolution of music copying beginning in the 1960s, when scores were primarily hand-written and professionally hand-engraved. In terms of professional work by composers and publishers, this remained generally true until the mid 1980s.
The earliest program to become widely available was the
SCORE program by Leland Smith at Stanford. This involved a specification language for musical notation written in FORTRAN (also used
to input music information into computer music systems), which controlled a pen plotting system ordinarily used to print out graphs and
maps from a computer. One wrote input code to control the pen; these
instructions were input on computer cards or on paper via Teletype
terminals. The program was versatile and accurate, but required some
dedication to master. (288; emphasis mine)
He notes an evolution as GUI-based computers became available and describes his own first use of electronic score-creation.
The first program I used to copy music was NoteWriter, developed
by Keith Hamel in the late 1980s to be used on a Mac computer. It
was basically a CAD (computer assisted design) program using music
In its article on scorewriters, Wikipedia also traces their commercial origins to the 1980s, and singles out SCORE as finding some level of adoption among publishers. Wikipedia also includes a full article on the software itself, dating it to 1967.2
Another reference to SCORE as the first -- "earliest" -- scorewriter, comes from the University of California Santa Cruz.
The Granddaddy of Music Programs
The earliest work on computer assisted notation was done by Leland C Smith at Stanford University (at the Center for Computer Research in Music and Acoustics, or CCRMA). Before precision computer printers were available, his SCORE program was creating complex music on an X-Y plotter. Music entry to SCORE was done on a teletype keyboard, and required an elaborate code for description of each element.
Leland continued to develop SCORE throughout his career, and it is now the standard program in use by the music publishing industry.3 It runs on PCs, and now features on screen graphic editing.
1 Robert Morris. "Calligraphy, Changes, Autonomy." Perspectives of New Music 52, no. 2 (2014): 284-96. Accessed December 16, 2020. doi:10.7757/persnewmusi.52.2.0284.
2 Wikipedia also includes a list of scorewriters, but without date information, and also seemingly without the earliest implementations. SCORE, for example, is not included.
3 This is likely no longer the case. Wikipedia's entry on SCORE indicates that SCORE has been "abandonware" since 2013 when its creator died.