The (fully canonical) factbook 'Ultimate Star Wars' is delightfully blunt.
Mace seems to be winning until Anakin suddenly intervenes and cuts off the Jedi Master's hand. Palpatine seals Mace's fate with a blast of Force lightning, sending him through the window to his death
The official "Revenge of the Sith" novelisation (by Matthew Stover) also states that he died:
Before he [Windu] could follow through on his stroke, a sudden arc of blue
plasma sheared through his wrist and his hand tumbled away with his
lightsaber still in it and Palpatine roared back to his feet and
lightning speared from the Sith Lord’s hands and without his blade to
catch it, the power of Palpatine’s hate struck him full-on.
He had been so intent on Palpatine’s shatterpoint that he’d never
thought to look for Anakin’s.
Dark lightning blasted away his universe. He fell forever.
The film's Junior Novelisation indicates that his body hit the ground with considerable force:
Another wave of Force lightning struck Mace and slammed him backward,
and back again, then it lifted him through the space where the window
had been, high into the night sky — and then let him drop the hundreds
of meters to the ground below. Anakin stared after him, horrified.
“What have I done?” he whispered.
The factbook "Star Wars in 100 Scenes" is also pretty unequivocal on Windu's fate.
The killing blow never lands. Anakin steps forward and severs Windu's
hand, allowing Sidious to blast his attacker and send him plummeting
from the building to his death. Anakin has made a fateful choice for
himself and the galaxy.
Out-of-Universe (just for a bit of fun)
The possibility of his having survived was jokingly discussed on the set of Star Wars III.
While Phillip Wade (Plo Koon) and Tay Bayliss (Stass Allie) are being
touched up by creatures department technicians. Silas Carson (Ki-Adi-
Mundi) and Jackson arrive. "You're a hologram from Mygeeto listening
to Yoda on Kashyyyk." Lucas says to Carson, who seems perplexed by the
director's relatively esoteric vocabulary.
"I had no clue what George was talking about." Carson would later
"Okay, the next shot is of Agen Kolar and Anakin," Fletcher says, and
the chairs are reoriented. While waiting, there's a fair amount of
joking that Mace Windu isn't really dead, and that, during the
credits, he could perhaps crawl back through the window, with his
clothes just a bit singed.
"Yeah, "Jackson says, "Mel Gibson could do it : The Resurrection."
Samuel Jackson (the actor who portrays Mace Windu) has remarked that he thinks that Mace survived the fall.
“I was trying to figure out, ‘Really? Can’t you just, like, injure
me—whatever?’ But in my mind, I’m not dead! Jedi can
fall incredibly high distances and not die.”
Apparently he shared this theory with George Lucas and was told in no uncertain terms that if he wanted to think that, that George had no problem with that.