Furniss joined US-headquartered Netcom when it launched in the UK in May last year, accompanied by David Clarke. Both had previously held executive posts at Compaq UK. Furniss was key figure in growing Compaq's channel when it launched its low-cost ProLinea PC range.
In a prepared statement, UK-based Demon said Furniss will look at "UK growth potential in the corporate and leased line marketplace".
Netcom claims to be the world's largest independent ISP but it has made slow progress in the UK and currently has 16,000 subscribers, a number dwarfed by Demon's 120,000 users.