Despite not getting out of second gear, Northampton raced into a 2-0 interval lead. Danny Jackman put them ahead when his 17th-minute free-kick beat Asmir Begovic via the upright.
Four minutes later top scorer Adebayo Akinfenwa grabbed his eighth goal of the season with a low drive from the edge of the box.
Abdul Osman went close to increasing Northampton's lead in the 28th minute when his close-range effort was cleared off the line by Kieran Murtagh. Yeovil rarely threatened but went close straight after the restart when Gavin Tomlin was denied by Frank Fielding's reflex save.
Karl Hawley should have added a third moments later when he headed straight at Begovic but Yeovil's hopes of getting anything from the game vanished when Terrell Forbes was dismissed in the 77th minute following a clash with Osman.
Jason Crowe finally added a third goal in the 82nd minute, rounding off a neat move to confirm Northampton's superiority.