He passed away last Sunday at the age of eighty-eight. Mooney's roots as an artist date back to Golden Age, where he worked for Timely/Marvel, the Igot/Eisner studio, and ghosted for Bob Kane on BATMAN.
In his post Golden-Age career, he's best known for his ten year stretch as the primary Silver Age Supergirl artist, his work on the Stan Lee penned AMAZING SPIDER-MAN, and his collaborations with Steve Gerber on OMEGA THE UNKNOWN and the MAN-THING.
He's been in semi-retirement since 1985, (although Wikipedia says he did some strange one-offs, like an issue of ELVIRA: MISTRESS OF THE DARK) but has been active on the convention circuit and his website states that he's still accepting commissions.
And from all accounts, and I've heard several, he was a hell of a nice guy. He will be missed.