They struck twice within a minute in the first half through Jason Scotland and Darren Pratley and after the interval Scotland netted his second from the spot.
The Swans were forced to make one change at the back with Alan Tate replacing the suspended Garry Monk.
A cross from Paul Anderson in the fifth minute was headed wide by Scotland who was in a good position in front of goal.
Anderson was causing the visitors' defence all sorts of problems and the Swans took the lead in the 16th minute after yet another sortie down the right flank by the Liverpool loanee.
The ball broke to Scotland and he hammered it in from close range for his ninth goal of the season.
Straight from the kick-off another pacey run down the right and some slick passing left the visitors' defence in ribbons and Pratley had time to pick his spot.
Northampton found it difficult to break out of defence, but when they did top-scorer Andy Kirk headed tamely at Dorus de Vries in the home goal.
Scotland should have bagged his second in the 39th minute but he failed to beat the keeper after running clean through.
The home side started the second half once again at pace and Andy Robinson had the ball in the net in the 50th minute only to be flagged offside.
After Robinson and Scotland both went close, the Swans went further ahead with a penalty from Scotland in the 66th minute after the referee spotted an infringement in the area.
Thomas Butler, who had replaced Robinson, continued the attacks down the flanks and by the final whistle the visitors had been run ragged.
Swansea passed the ball around like a top of the table side and if they keep this form going then promotion favourites Leeds and Forest will know they're in for a tremendous fight at the end of the season.