There are 35 magnetic strips inside a Mellotron, one for each key, which store analog samples of acoustic instruments, e.g., violins, flutes or voice. The strips are mounted unrolled in a complicated arrangement of pulleys and springs. Depressing a key engages a capstan that pulls the tape across a magnetic read head. The length of the note is limited to eight seconds. However, because the tape has a clear beginning and end, it's possible to record and play back the attack at the start of the note, e.g., the "pluck" of a guitar string. This gives the Mellotron an eerily realistic sound. Each strip has three tracks and so can store three instruments; the track is changed by shifting the entire set of tapes mechanically.

Although the Mellotron was too difficult to maintain for its intended market—home and institutional use—it was adopted by rock musicians in the 1960s, starting with the Moody Blues (Nights in White Satin) and The Beatles (Strawberry Fields Forever), and went on to become a staple of progressive rock with groups like King Crimson, Yes and Genesis. In the 1990s and beyond there was a revival of interest among performers like Oasis, Red Hot Chili Peppers, The Smashing Pumpkins and Radiohead.

In 1970, the Mellotron M400 introduced a tape frame that could easily be removed and replaced by one with different voices. Changing a tape frame took less than two minutes. But the instrument was temperamental and required frequent adjustment. It was affected by changes in temperature. Tapes got stuck in the middle of performances. To quote Tony Banks, keyboard player for Genesis: "I have to be honest: when I could stop using the Mellotron, it was a great weight off my mind" (Banks 2008).

The Mellotron was based on a device invented in the United States in 1949 by Harry Chamberlin, who recorded the samples with members of the Lawrence Welk Orchestra. One of Chamberlin's salesmen took the design with him on a trip to England and presented it to another company as his own work. Litigation, of course, followed. Eventually, licensing deals were arranged and both companies continued to manufacture their version. Both companies went out of business in the 1980s with the availability of polyphonic synthesizers and samplers.