Forsyth grew up in Dundee where he attended the High School of Dundee and his first writing appeared in a Dundee United fanzine. He graduated from Edinburgh University and held several jobs including as a nightclub promoter before working as a freelance journalist. He is also a graduate of the New York Film Academy.
Forsyth's first book Other People's Money (2007) told the true story of the Scottish fraudster Elliot Castro. It received significant coverage and was well received  though some newspapers questioned the book's moral purpose. "They thought it was wrong that we should profit from Elliot's crimes," Forsyth said, "But that's always something I protested against quite vigorously. Elliot was caught, and sentenced, and paid for his crimes." The film rights were purchased by the producers of The Last King of Scotland. In 2014 it was reported the book is being adapted for film by Crabtree Films with Forsyth writing the screenplay.
Forsyth has written four books featuring the fictional comic character Bob Servant: Delete This At your Peril (2007), Hero of Dundee (2010), Why Me? (2011), and Ask Bob (2015). In 2009 Irvine Welsh selected the then out of print Delete This at Your Peril as his choice in an Esquire Magazine poll for the Funniest Books Ever. Upon the book’s reissue, Barry Fantoni, wrote “'I have worked with a lot of funny men - Peter Cook, Spike Milligan, Harry Enfield. Bob Servant is in a class of his own”. In 2011, The Scotsman said “Bob Servant has attained national treasure status” while the Press and Journal called Bob Servant “a modern Scottish comedy classic.” 
Forsyth has noted the Dundonian poet William McGonagall as an influence for the character along with Harry Flashman. The most recent Bob Servant book, Ask Bob, is a compilation of readers’ questions and included letters from fans Sir Chris Hoy, Guy Berryman, Irvine Welsh, John Niven, Barry Fantoni and Jack Whitehall.
After watching a medium perform in Edinburgh, Forsyth wrote the novel Let Them Come Through (2009). Forsyth researched the psychic world by attending live shows and speaking to experts including James Randi. Let Them Come Through was published in the UK and the United States and was praised for Forsyth's dark humour. Forsyth’s second novel, San Carlos (2014), is set in Ibiza in the 1980s and is a thriller following a reformed neo-Nazi seeking a new life. San Carlos was Book of the Week in the Daily Mirror while The Herald said that Forsyth had shown a “different side to his talents” in the “pacy, unpretentious thriller.” 
Praise for the Bob Servant books along with support from a number of bands including Snow Patrol and Belle and Sebastian, created a word of mouth campaign that attracted the attention of the BBC. Forsyth first adapted Bob Servant for Radio Scotland in The Bob Servant Emails (2012), starring Brian Cox as Bob Servant and Felix Dexter, Laura Solon, Sanjeev Kohli and Lewis Macleod. Forsyth attracted Cox to the project after meeting a friend of the Dundonian actor in a pub in New York. Cox said "We've had Billy Connolly and Lex McLean's Glasgow stories and traditions, but now we've got this Dundee creature".
Bob Servant made the leap to television in early 2013 in the cult BBC Four comedy Bob Servant Independent which added Jonathan Watson and Rufus Jones to the existing radio cast. The series was well-received and a second series (renamed Bob Servant) was broadcast in 2015 on BBC Four across the UK and BBC One in Scotland. The second series won the Royal Television Society Scotland award for Comedy.
In 2016, Forsyth wrote two one-off Playhouses for Sky. Elizabeth, Michael & Marlon was based on the apocryphal story that on 9/11, Marlon Brando, Elizabeth Taylor and Michael Jackson hired a rental car and drove from New York to Ohio. The Playhouse was shot in late 2015, starring Brian Cox, Elizabeth Taylor, Joseph Fiennes. It also featured Carrie Fisher in one of her final roles. Before the planned 2017 transmission of the show adverse reaction to the casting of Fiennes as Jackson, particularly from Jackson’s daughter Paris Jackson, saw the show controversially pulled by Sky.
Forsyth’s other Sky Playhouse, Waiting for Andre, concerned the real-life friendship between Samuel Beckett (David Threlfall) and a teenage Andre the Giant (Liam Macdonald). The Playhouse was transmitted in January 2017 to positive reviews. Observer called it a “gorgeous and sumptuous half-hour” and noted it was a “great shame Beckett himself didn’t get to enjoy this delightful slice of life.”.
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