Like all the shows since Four Feather Falls, the music was naturally provided by Barry Gray. Gray had also provided the themes for The Adventures of Twizzle and Torchy The Battery Boy, but was yet to reach the standards of the puppet and live-action shows. Born in 1908 as John Livesey Eccles, he would provide exceptional standards of work to his shows on all the Anderson shows he did; from the Thunderbirds march to the abstract drumbeat of Captain Scarlet, from This Is The Twist from Fireball XL5 to the Dangerous Game in The Cham Cham, from the upbeat theme of Joe 90 to the grand opening of Space 1999. The Secret Service theme music however was reportedly his favourite of all the themes he composed, you could tell it was. Originally seeking the vocals of the Swingle Singers, he eventually approached the Mike Sammes Singers for the shows opening and closing titles. They recorded on 16th October 1968 in a four-hour studio session.
The first episode (A Case For The BISHOP) was recorded at Olympic Studios in London on 12th November 1968 with 29 musicians with vocals from the Mike Sammes Singers. A Question Of Miracles took recording on 11th December at Gray's private studio with eight musicians. With the series cancelled, it would only have seemed logical to score only a couple of more episodes, which were The Feathered Spies and Last Train To Buffler's Halt, recorded on 10th January 1969. As a result, tracks would be drawn up from earlier Anderson shows. A track from the Supercar episode Amazon Adventure can be heard in Errand Of Mercy, tracks from the Stingray episode The Golden Sea can be heard in A Question Of Miracles, The Feathered Spies, Errand Of Mercy, Recall To Service and The Cure. The Captain Scarlet pop theme, a variation of the radio soundtrack of the satellite TVR-17 in White As Snow, is heard in School For Spies and More Haste, Less Speed. Many tracks from Joe 90 are reused throughout the series with tracks from the episodes Hi-Jacked, Three's A Crowd, International Concerto, Big Fish, King For A Day and Arctic Adventure.
However, it would also mean that tracks from The Secret Service would also be used for future Anderson programs UFO and Space 1999, the last shows Barry Gray would score original scores for Gerry Anderson. Tracks from The Feathered Spies be heard in the UFO episode A Question Of Priorities, with tracks from Last Train Train To Buffler's Halt appearing in numerous episodes like ESP, The Sound Of Silence, The Cat With Ten Lives, Reflections In The Water, Timelash and Mindbender. In Space 1999, tracks from A Case For The BISHOP can be heard in Missing Link and The Last Sunset, with music from Buffler's Halt appearing in episodes like Missing Link, Alpha Child, Voyager's Return, Collision Course, War Games, The Last Enemy, Space Brain and Mission Of The Darians. The soundtrack for The Secret Service was released in 2007 exclusively for Fanderson members (the Gerry Anderson appreciation society).