Mistakes by other drivers, notably Charles Leclerc and Max Verstappen, left Magnussen in a surprise fourth place after the first runs in the last segment of qualifying.
A crash for Leclerc brought out the red flag and ensured that nobody could improve their times on their second runs and demote Magnussen.
He then survived an impeding investigation after an incident with Lewis Hamilton early in Q1, securing the position for his US-owned team.
"It feels good," said the Dane. "It's an important race for us. Home race for the team, all our big sponsors are here this weekend.
"And this is a race we hope goes well, it's one of those. I got a bit lucky. But I don't feel bad about it."
Magnussen, who survived a spin and brush with the wall in FP2 without damage, said that the car – which is running a floor upgrade – had been good from the start of track action.
"I think this weekend we've just been strong consistently," he noted. "And not only in terms of putting in lap times, and if you look at the results from each session, it's so good.
"But just the feeling was there from the very beginning. We put the car on track and it worked. We haven't really touched it.
"And I think that's been really good. And then we brought an upgrade that worked, so yeah, happy with P4 tomorrow, and hopefully we can finish it off and score some points."
Source: Autosport