Hakkinen won his two World titles with McLaren in 1998 and 1999 when they were powered by Mercedes up until the end of 2014.
They have since used Honda and Renault engines the past few seasons but have failed to replicate the success they once had before, opting to return to being supplied by the Silver Arrows.
"It is very satisfying to see McLaren return to a position of relative competitiveness, and this week's news that they use Mercedes engines from 2021 onwards is very significant," Hakkinen wrote in his Unibet column.
"I won both my World Championships for McLaren-Mercedes, and this week is the 18th anniversary of my 20th and final victory for McLaren.
"It is a partnership that worked very well in the past, and team boss Andreas Seidl has identified this as being a key element in McLaren's strategy to become winners again.
"The 2019 season is not even finished but already Formula 1 is turning its attention to 2020 and even 2021, when new regulations should make it possible for more teams to compete to win.
"McLaren's determination to join Mercedes, Ferrari and Red Bull Racing in battling for victories is important, both for them and for Formula 1."