Lahiri, starting the day at five-under and in tied-seventh place, slipped to tied 28th at two-over.
He was done in by a five-hole stretch from third to seventh holes, when he bogeyed all five of them, finding either rough or one of the bunkers.
With the wind blowing 15-20 mph, Trump National Doral began to show its teeth and played the toughest it had all week, nearly a full two strokes over par.
Lahiri will now take a break and return to India to defend his Hero Indian Open title. After which he would go back to USA for the PGA Tour and the Masters next month.
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Meanwhile, the 35-year-old Adam Scott, continued his blistering run, as he added the WGC-Cadillac to make it two wins in two weeks.
Scott, who finished second in Northern Trust Open and then won the Honda Classic last week, maintained his form and moved to No. 6 in the world. He shot 69 to finish at 12-under 276 for the week and moves to the top of the FedExCup standings with the win.
It was also his 13th PGA Tour win.
Bubba Watson (68) was second, one shot behind at 11-under, while Danny Willett (69) and McIlroy (74) were tied third at 10-under and Phil Mickelson (70) was fifth at nine-under.
Lahiri opened with a pair of pars before running into rough weather when he found only 50 per cent of the fairways and found a lot of bunkers and roughs. He managed to scramble successfully only once in five times and missed a few putts too.
Scott went from three down at the start of the afternoon, to two down after one hole, to six back after five and finally when the dust had settled over Trump National Doral he was right on top and one ahead of Watson.