The computer programmer who implemented the first email program on the ARPANET system was Ray Tomlinson. In 1971, he developed the software that allowed users to send and receive electronic messages between different computers, thus pioneering the concept of email as we know it today. Tomlinson is widely credited with introducing the use of the "@" symbol to separate the user name and the host name in email addresses, a convention that has endured since then. His contribution revolutionized communication and laid the foundation for the widespread use of email across the internet.