The victory handed Mertens her sixth Grand Slam title while Zhang won her third but the triumph almost never happened, with the Belgian-Chinese pair forced to save three match points in the second round against teenagers Iva Jovic and Victoria Mboko.
"It's been quite a week so far," said Mertens, who is set to move up to world number one in the doubles rankings.
"I'm lost for words. In the second round of doubles, we had three match points against us. We kept on fighting, we kept on believing and we really grew as a team. Today, in the final, I'm happy that I served it out eventually.
"It's the first week that we've actually played together and what a result. It's been amazing."
Danilina and Krunic had a superb start when they raced to a 4-1 lead in a gripping opening set but the momentum turned gradually as Mertens and Zhang clawed their way back, winning four games in a row.
As Danilina and Krunic suddenly found themselves trailing, they responded with renewed composure, playing solidly from the baseline and showing sharp movement at the net.
Holding their nerve under pressure, the pair secured two crucial games to stay alive in the set and successfully force a tiebreak.
Title-winning experience shines through
But the experience of Mertens and Zhang shone through in the tiebreak, with the Belgian underlining her class on a standout point, producing a flurry of quick-reaction shots to bring up set point before the pair wrapped up the set in 55 minutes.
The fourth seeds carried that momentum into the second set to take a commanding 5-0 lead with assured net play and well-placed winners before Danilina and Krunic managed to get on the board.
Although they fought back and saved two championship points as Krunic lit up the Rod Laver Arena, the fourth seeds sealed victory on Mertens' serve when Zhang fired a backhand winner at the net.
"We had a lot of things we went through this past couple of months. I had a couple of things I had to deal with at home and it wasn't easy for us," Krunic said.
"But I'm very proud of us, that we found a way to get here, to give ourselves the best chance. I think that's the best we can do and we learn from every loss.
"Out of three Grand Slams together, two finals is a pretty damn good job."