Garner worked as a "Pioneer Evangelist" for the Wilson Carlile College of Evangelism, a centre for the training of Christian evangelists, run by the Church Army, an organisation within which he held the rank of Captain. In February 2007, he wrote "A Call for Apostles Today" (ISBN 9781851746477), which argues that individual apostles, and groups, working in parallel with churches, are a more effective way of promoting Christianity than are churches alone. He is also the director of the Free Spirit Trust, an organisation promoting Christian missionary work, and jointly runs the "Great APEs" website, a website for "Apostles, Prophets and Evangelists".
His ornithological writing has included articles for British Birds and Birding World magazines. He is credited with discovering the first British records of Caspian Gull, in Essex in the 1990s. In 1997, with David Quinn and Bob Glover, Garner published a two-part paper in British Birds covering the identification of Yellow-legged and Caspian Gulls, which covered the former in far greater detail than any previous published work, and contained the first detailed English-language descriptions of the latter. His first bird-related book, Frontiers in Birding (ISBN 9781898110477), was published by BirdGuides Ltd. in 2008. He is a member of the British Birds Rarities Committee.
- Grove Books profile of Martin Garner (accessed 22 June 2008)
- Martin Garner's Church Army people directory entry (accessed 22 June 2008)
- Free Spirit Trust website (accessed 22 June 2008)
- Great APES website (accessed 22 June 2008)
- Rogers, M. J. and the Rarities Committee (2003) Report on rare birds in Great Britain in 2002 British Birds 96(11): 542-609 (section on Caspian Gulls is on pages 575-8)
- What is a Caspian Gull?, Dick Newell, 17 January 2003, Birdguides (accessed 17 August 2008)
- Garner, Martin, illustrated by David Quinn (1997) Identification of Yellow-legged Gulls in Britain British Birds 90(1) 25-62; Garner, Martin, David Quinn and Bob Glover (1997) Identification of Yellow-legged Gulls in Britain, part 2 British Birds 90(9): 369-83
- BirdGuides page describing Frontiers in Birding (accessed 22 June 2008)
- Dean, Alan R. (2007) The British Birds Rarities Committee: a review of its history, publications and procedures British Birds 100(3): 149-176