Tomlinson was born in Amsterdam, New York, but his family soon moved to the small, unincorporated village of Vail Mills, Broadalbin, New York. He attended Broadalbin Central School in nearby Broadalbin, New York. Later he attended Rensselaer Polytechnic Institute (RPI) in Troy, New York where he participated in the co-op program with IBM. He received a bachelor's degree in electrical engineering from RPI in 1963.
After graduating from RPI, he entered the Massachusetts Institute of Technology (MIT) to continue his electrical engineering education. At MIT, Tomlinson worked in the Speech Communication Group and developed an analog-digital Hybrid speech synthesizer as the subject of his thesis for the master's degree in electrical engineering, which he received in 1965.
In 1967 he joined the Technology company of Bolt, Beranek and Newman (now BBN Technologies), where he helped develop the TENEX operating system including the ARPANET Network Control Program, implementations of Telnet, and implementations on the self-replicating programs Creeper and Reaper. He wrote a file transfer program called CPYNET to transfer files through the ARPANET. Tomlinson was asked to change a program called SNDMSG, which sent messages to other users of a time-sharing computer, to run on TENEX. He added code he took from CPYNET to SNDMSG so messages could be sent to users on other computers—the first email.
Tomlinson said he preferred "email" over "e-mail", joking in a 2010 interview that "I'm simply trying to conserve the world's supply of hyphens" and that "the term has been in use long enough to drop the hyphen".
Tomlinson died at his home in Lincoln, Massachusetts, on March 5, 2016, from a heart attack. He was 74 years old.