The title, which has ranked in ABC’s last audit as the most popular UK online paper for 13 months in a row, has surpassed AOL-owned The Huffington Post with 39.6m global users for March.
The Huffington Post ranked third, according to Comscore, with 38.4m users for March, while The New York Times led with 61.9m.
The figures will be a coup for Mail Online, which has been building its US presence for around a year. It kicked off an aggressive push in the country last year, taking on editorial staff at its Los Angeles office to produce dedicated US content.
Guardian.co.uk, which has also recently made moves to expand into the US, deploying guardian.co.uk editor Janine Gibson to New York to head Guardian News & Media’s digital operation launching there later this year, has seen significant growth, from 25.2m in February to 30.8m in March, according to Comscore.
Her appointment follows that of former FT president Steve Howe, who was hired as chief revenue officer.
The Telegraph, Guardian and Independent have all partnered, at some point, with overseas ad sales house AdGent 007 to help monetise overseas readers.
The Guardian launched guardianamerica.com in 2007 to cater for its US audience.
The Huffington Post is looking to launch a UK edition later this year, as part of a wider European expansion strategy. The US-based news and analysis site was bought by AOL in February for $315m (£199m) in a deal that saw the website’s founder Arianna Huffington join the company as editor-in-chief and president of a new unit of AOL, Huffington Post Media Group.