Chasing a target of 232, which Essex had reached the previous day largely due to a brilliant knock by the teenager Stephen Peters, Middlesex had their chances, most notably when Justin Langer and Owais Shah were in full flow.
Coming together after Cowan had reduced the home side to 51 for 3, they added 108 runs before Cowan removed the pair with the first and fourth balls of his second spell, both batsmen walking across straight balls to be lbw. It was the point that swung the tie in Essex's favour, a position the visitors succeeded in holding despite threatening flurries from Jason Pooley and Paul Weekes.
Like Ben Hollioake, Cowan appears to have something of an affinity for Lord's. Last September he took 3 for 29 here in the NatWest final, a performance that helped secure his selection for the West Indies.
But if that tour proved disappointing, at least from a playing point of view - he took 1 for 200 in three tour matches - it at least brought him the benefits of working cheek by jowl with Angus Fraser. A confirmed non-swinger, Fraser has helped Cowan tighten his line and length, a factor that helped undo Middlesex almost as much as his swing.
Ironically, these were Cowan's first wickets in the competition this season, the start of which has proved frustrating, following an injury to his side that has kept him out for three weeks. Middlesex will probably not want to be reminded, but this was his first game back.
Using the slope from the Nursery End to accentuate his natural outshape, Cowan removed both openers, the second, Keith Brown, falling to a stunning diving catch at second slip by Stuart Law. Essex's Australian has been relatively muted this season, but that catch plus several nerveless overs at the death all played their part.
Cowan's double strike brought the the Middlesex captain Mark Ramprakash to the crease. In fine form after his two hundreds in the Championship at Uxbridge, Ramprakash's second scoring shot was a soaring six into the Mound Stand off Mark Ilott.
The poise was to prove illusory and, pushing tentatively at an outswinger, he edged Cowan low to Law, still at slip. With the skipper gone, Langer went about a repair job like an expensive crimper, cutting and clipping carefully while Shah found his feet.
With both batsmen surviving difficult chances, however: Langer on 13 and 47; Shah on 16, the pair looked to have judged their retort perfectly. It was not to be, and, in the space of four balls in the 38th over, Cowan, did the crucial damage.
BENSON AND HEDGES CUP Semi-final draw: Leicestershire v Surrey; Yorkshire v Essex. (Ties to be played 9 June).Reuse content