Millwall's hopes of revitalising their flagging league campaign suffered an early blow when Simon Cox, the Swindon striker on loan from Reading, capitalised on some poor defending to put Paul Sturrock's men ahead.
With seven minutes gone, full-back Jack Smith was allowed to collect a short pass from Lee Peacock, drive unobstructed into the area and deliver a low far-post cross to Cox, who turned the ball home with minimal fuss.
Millwall's response was limited and, two dubious handball claims aside, the closest they came to a first-half equaliser was when Dave Brammer drove a speculative long-range effort over the bar.
With little to concern them at the back, Swindon sought to double their tally and would surely have done so had Billy Paynter got more purchase on his shot after turning his marker on the edge of the six-yard box. Cox then tested Robert Douglas from range, prompting a decent save from the Lions goalkeeper.
Millwall left the field at the interval to a chorus of boos, but a half-time switch to 4-5-1 gave them a new lease of life.
Chris Hackett replaced Ryan Smith on the right wing and Jay Simpson dropped into midfield to leave Will Hoskins ploughing a lone furrow up front.
The change reaped virtually instant dividends. With 56 minutes gone, Ali Fuseini's neat touch brought a high ball under instant control, providing him with the vital second necessary to find Simpson, who raced clear of the visiting defence to slot calmly past Smith.
But 12 minutes from time, Ifil met a Sofiene Zaaboub corner at the near post to prod home from close range and condemn the home side to their fifth defeat in six league games.