Mew was born in Newport, Isle of Wight, and started swimming at West West Swimming Club under the guidance of Trevor Collins; he stayed until he moved to the High Performance Center, then located at the University of Bath, at 17. Before he moved Mew competed in three successive European Junior Swimming Championships – 1995 in Geneva, Switzerland, 1996 in Copenhagen, Denmark and 1997 in Glasgow, United Kingdom, winning three medals in total. During this time he also broke the 50, 100 and 200 metre breaststroke long course and 50 and 100 metre breaststroke short course junior British records.
Within six months of his last Junior European Swimming Championships he made his senior debut at the 1998 World Aquatics Championships in Perth, Australia. Later that year he won two medals at the 1998 Commonwealth Games in Kuala Lumpur including a bronze in the 100 m breaststroke.
Mew trained at The Race Club, a summer swim camp founded by Olympic swimmers Gary Hall, Jr. and his father, Gary Hall, Sr. The Race Club, originally known as "The World Team", was designed to serve as a training group for elite swimmers across the world in preparation for the 2000 Sydney Olympic Games. To be able to train with the Race Club, one must either have been ranked in the top 20 in the world the past 3 calendar years or top 3 in their nation in the past year. The Race Club included such well known swimmers as Roland Schoeman, Mark Foster, Ryk Neethling and Therese Alshammar.
Mew has swum in two Olympic finals, the 4×100 m medley relay at the 2000 Summer Olympics and the 100 m breaststroke at the 2004 Summer Olympics. Unfortunately Darren ripped the sheafing that keeps the ulnar nerve in place in his left elbow three months before the Athens Olympics leaving him with limited strength in his left arm; this was operated on shortly after he competed at the Games.
He has also won two medals at the FINA World Swimming Championships (25 m), four medals at Commonwealth Games six medals at the European Short Course Swimming Championships, and one medal at the 2007 Summer Universiade.
He has also broken multiple English, British, and Commonwealth records, and in April 2004 set the fourth fastest 100 metre breaststroke time in world history. From 1997 over the next ten years he had a world ranking inside the top ten in the world in either short course or long course breaststroke swimming.
In 2000, with a bonus from a sponsor Darren set up the "Darren Mew Trust Fund" to help support sports clubs and teams on the Isle of Wight. The Fund helped support clubs and teams by helping with travel expenses and buying new equipment.
In 2009, Darren was part of the "Champions" project by photographers Anderson & Low to help raise funds for Elton John AIDS Foundation (EJAF). The project which included famous sports people such as Thierry Henry, Matt Dawson, Mark Foster, Billie Jean King and Venus Williams were pictured in the nude. These pictures were made into a book "Champions" and were also displayed in the National Portrait Gallery in London.