Connacht paid a big price for failing to take chances in difficult conditions at the Sportsground as they fell to a disappointing defeat.
They created the opportunities to win it but the accuracy that enabled their superb win in Edinburgh last time out was absent from a side whose last two games were called off due to Covid issues with Benetton and Dragons.
Scarlets led 13-7 at the break having used the strong breeze in the opening half to kick for good territory and score identical tries in the right corner.
Ryan Conbeer, the former Welsh U20 winger, scored both of them, giving them a good start after seven minutes when Scarlets created an overlap across the field from the left with winger Steff Evans swinging around to put his opposite number in at the right corner.
Connacht hit back 16 minutes in when lock Gavin Thornbury retrieved a lost lineout and with Ultan Dillane getting a good drive, they were stopped short of the line before No.8 Abraham Papali’i crowned his home debut by looping off a maul to score under the posts.
Jack Carty added the conversion but ten minutes from the break he spilled a swirling clearance from Evans and from the resultant lineout, Scarlets again created an overlap as they went across from the left for Conbeer to score despite tackles from John Porch and Alex Wootton.
Dan Jones again missed the conversion but he added a penalty from 20 metres to leave them six ahead at the break.
Scrum-half Dane Blacker finished off a breakaway try three minutes after the restart to leave Connacht with a mountain to climb at 20-7 adrift.
Blacker got back to deny Porch in the left corner and then captain Steff Hughes did likewise to dispossess Matt Healy just as he was about to score after 57 minutes.
But Connacht got hope heading into the final quarter when a drive off a scrum by Paul Boyle was stopped short but quickly recycled and Dillane got the touchdown after being driven over by his second-row partner Thornbury. Carty converted to make it 20-14.
Connacht had chances to snatch it, the closest coming six minutes from time after a series of drives in the right corner but Denis Buckley was pinged for a knock-on over the line and Scarlets escaped with their third win of the campaign.