The prediction was contained in Gartner's latest forecast of device shipments which it released to Computerworld.
The forecast said that in 2017, devices powered by macOS and iOS would push Windows devices into second place. This gap would then widen in the next two years.
According to the figures, of the 2.3 billion devices shipped in 2016, Windows powered about 260 million. Google's Android mobile operating system was by far the leader.
Gartner said that Apple would recover in 2017 and add another 8% to increase the number of devices its operating systems powered to 268 million. A further 3% would be added in 2018 (276 million) and another 3% increase would be seen in 2019 (285 million).
The research firm forecast that Windows devices shipped in 2017 would fall to 252 million, a year-on-year drop of 3%.
This would then marginally increase to 253 million in 2018 and show a bigger increase the following year (257 million).
The gap in device shipments, which was 12 million in 2016 in Microsoft's favour, would thus blow out to 28 million in Apple's favour by the end of 2019, Gartner predicted.
As far as total shipments of PC-like devices go, Gartner said they were 268 million for 2016, 7% lower than in 2015.
It predicted further declines in 2017, dropping 0.7% to 266 million. Improvement was seen in 2018 (272 million) and 2019 (278 million), with a sub-300 million figure being the new norm.