Plain, 36, from Albury, Australia, reached the 8,850-meter peak of the world's tallest mountain after climbing more than seven hours from the final camp, at the 8,000m South Col, to claim the record.
A Polish climber held the previous record of 126 days to complete the Seven Summits.
The other six highest peaks are Denali (North America), Elbrus (Europe), Vinson (Antarctica), Aconcagua (South America), Kilimanjaro (Africa) and Papua New Guinea's Carstensz Pyramid (Australasia/Oceania).
More than 340 foreigners, each paying US$11,000 (HK$85,800) for a climbing permit, and their sherpa guides are waiting to tackle Everest from the Nepal side. In Tibet, 180 climbers are waiting to ascend.