Boothroyd was born in Leeds. His family moved to Scarborough in 1976 where he attended Newby County Primary School, and to Congleton, Cheshire, in 1983 where he attended Quinta County Primary School, and from 1984, Heathfield High School. From 1989 to 1991 he attended The King's School, Macclesfield, a public school/independent school. He then went to St. Catharine's College, Cambridge University, where he read Natural Sciences, and was involved with the Cambridge Organisation of Labour Students; among the other members at this time was Andrew Gilligan.
Having already joined the Labour Party in 1989, on leaving university Boothroyd worked briefly for John Battle, MP for Leeds West. Boothroyd was active in local politics in Westminster City Council and fought unsuccessfully for a seat on it in 1998. He started a website, http://www.election.demon.co.uk, which gives UK election results, and wrote a book called Politico's Guide to the History of British Political Parties in 2001, which gives potted histories for the smaller fringe parties and longer histories for the larger ones. He also worked as a research assistant for Ian Lucas, MP for Wrexham, after the 2001 general election.
He is currently Head of Research for Indigo Public Affairs, a company specialising in urban regeneration schemes and lobbying for planning consent. There he edits the company elections blog and councils list.