Percy Cox was born in Gresham, Norfolk in 1879, and trained as an engineer in London. He was an apprentice alongside Henry Pearson at the General Engine and Boiler Company in Pomeroy Street, Bermondsey. It was there that he met his wife, Nellie Louise Wheelwright, the sister of another apprentice. They married in 1906.
Cox and Pearson went into business, and worked together between 1906 and 1916 to produce steam cars and motorcycles, selling their first steam bicycle in 1912. Business ceased after the development of petrol engines during World War I and after Henry Pearson's death during the War. Cox then moved on to other ventures, going into business with Robert Temple. Together they developed a humane slaughtering device, the stun gun. After Temple's death, Cox went into business with his son Morris ("Tim") and daughter Dora, developing the Cox Submarine Gun in time for World War II.
Percy Cox died in Keston, Kent on 08 Aug 1942, aged 63. Nellie Cox died in the early 1950s.