Simon originally studied fashion and textile design  and later taught himself to cook. In 1990, he bought Greens, a vegetarian restaurant in West Didsbury, Manchester in 1990. Simon opened his second restaurant, Earle, in Hale in October 2006, though this was later sold to a local restaurateur in 2016. In 2013, Simon served as a judge for British Sausage Week, helping to choose the winners and promote the event. 

Simon's television career began with Granada Breeze before appearing regularly on programmes including This Morning, The One Show, The Gadget Show & Pointless Celebrities. He has presented Making a Meal of It for BBC Two, Win It, Cook It, Tricks of the Restaurant Trade, Eat the Week with Iceland  & Secrets of Our Favourite Snacks  for Channel 4 & Breaking Into Tesco for Channel 5. Rimmer is, however, best known for his work alongside Tim Lovejoy with whom he presented Something for the Weekend on BBC Two between 2006 and 2012  & Sunday Brunch, a show with a similar format on Channel 4. 

Simon has also appeared as a contestant on various TV shows. In 2006, he pitted his cooking talents against other skilled chefs on the first series of Great British Menu; losing in the North of England heat to Marcus Wareing. In 2008, he appeared on Celebrity Mastermind, choosing 'The History of Tranmere Rovers' as a specialist subject. In August 2017, it was confirmed that Rimmer would take part in the fifteenth series of Strictly Come Dancing, partnered with Karen Clifton. The pair were eliminated in week six. 

Simon's first book, The Accidental Vegetarian, was published in October 2004. His second book, Rebel Cook, was published in October 2006. His third book Lazy Brunch came out in spring 2008, co-written with Lovejoy and based on a feature in Something from the Weekend. In spring 2009, Rimmer published his fourth book, The Seasoned Vegetarian.