Online retailer Amazon has been a prime beneficiary of the growth of holiday shopping online, and of procrastination.

Amazon Prime, the company’s $99-a-year loyalty program that includes unlimited free two-day shipping, gained 3 million new members in the third week of December alone. Meanwhile, the number of Prime members shopping via mobile more than doubled this holiday season, Amazon said Monday.

Christmas Eve was the biggest day yet for deliveries by the company’s Prime Now service, which now offers free two-hour delivery to more than 20 metropolitan areas. When Prime Now launched last year, it only delivered within Manhattan.

Seattle-based Amazon also said it shipped a record number of items worldwide for the holidays, with items going to 185 countries.

The 20-year-old company is profiting from major trends reshaping holiday shopping in the U.S.

More and more people do part or even all of their shopping online, increasingly using mobile devices such as tablets and smartphones to “window shop” and then buying items from their couch or their desk at work. Retailers are deluging consumers with e-mail offers for online-only deals.

And procrastinators, once stuck dealing with huge crowds at shopping malls and big-box stores right before Christmas, now can do last-minute shopping online. Amazon and some other retailers promised that items ordered as late as the morning of Christmas Eve would arrive by midnight.

advertisement | advertise on newsday

Amazon’s report issued on Monday covers trends and percentage changes in its shipments, but doesn’t give sales figures or the number of items it shipped. Amazon.com Inc. declined to provide additional information.