The earliest documented spam (although the term had not yet been coined) was a message advertising the availability of a new model of Digital Equipment Corporation computers sent by Gary Thuerk to 393 recipients on ARPANET on May 3, 1978. Rather than send a separate message to each person, which was the standard practice at the time, he had an assistant, Carl Gartley, write a single mass email. Reaction from the net community was fiercely negative, but the spam did generate some sales.

Spamming had been practiced as a prank by participants in multi-user dungeon games, to fill their rivals' accounts with unwanted electronic junk.

The first major commercial spam incident started on March 5, 1994, when a husband and wife team of lawyers, Laurence Canter and Martha Siegel, began using bulk Usenet posting to advertise immigration law services. The incident was commonly termed the "Green Card spam", after the subject line of the postings. Defiant in the face of widespread condemnation, the attorneys claimed their detractors were hypocrites or "zealouts", claimed they had a free speech right to send unwanted commercial messages, and labeled their opponents "anti-commerce radicals". The couple wrote a controversial book entitled How to Make a Fortune on the Information Superhighway.

An early example of nonprofit fundraising bulk posting via Usenet also occurred in 1994 on behalf of CitiHope, an NGO attempting to raise funds to rescue children at risk during the Bosnian War. However, as it was a violation of their terms of service, the ISP Panix deleted all of the bulk posts from Usenet, only missing three copies.

Within a few years, the focus of spamming (and anti-spam efforts) moved chiefly to email, where it remains today. By 1999, Khan C. Smith, a well known hacker at the time, had begun to commercialize the bulk email industry and rallied thousands into the business by building more friendly bulk email software and providing internet access illegally hacked from major ISPs such as Earthlink and Botnets.

By 2009 the majority of spam sent around the World was in the English language; spammers began using automatic translation services to send spam in other languages.