Shaun Goodhall scored a consolation for Town after Cardiff had taken a 4 - 0 lead.
Northampton Town: Guest, Danny May, Freddie Avill, Alex Dyer, Nick Thorogood, Brett Johnson, Greg Taylor, Joe Burnell, Shaun Goodall, Ryan Gilligan, Joe Benjamin.
Cardiff City: David Forde, Chris Gunter, Darren Purse, Anthony Anchenpong, Ashley Buswell, Kevin Cooper, Peter Whittingham, Aaron Ramsey, Darcy Blake, Steven Thompson, Terrell Webb.