The one-man band of showbiz passed away from natural causes Thursday at his home in Beverly Hills -- this according to Deadline. No other details of his death were released.

Shapiro cut his teeth in entertainment by working as a hungry talent agent -- surrounding himself with some of the biggest names in entertainment at the time ... including Elvis Presley and Carl Reiner, the latter of whom he went on to represent/manage personally.

Eventually, he started his own business -- and while he continued to manage certain talent, like Kaufman, most of his work seemed to focus on producing hit TV shows ... some of which include "The Steve Allen Show," "That Girl" and 'Gomer Pyle.'

He also packaged a number of specials for huge variety show hosts at the time, like Dick Van Dyke, Mary Tyler Moore and Carol Channing. But, his biggest contribution to TV would come in the '90s. Of course, we're talking "Seinfeld," for which he served as an EP.

Jerry and George remained very close after the sitcom's epic run -- GS even went on to produce a number of standup specials for Jerry, and memorably appeared in the 2002 doc "Comedian" ... where Jerry tries his hand at comedy again in the wake of "Seinfeld."

As involved as he's been with Jerry over the years -- including continuing to produce for JS's recent projects, like "Comedians in Cars Getting Coffee" -- Shapiro was equally devoted to the late, great Kaufman ... going through the rollercoaster of his notoriety back in the day.

Their relationship was dramatized on film in 1999's 'Man on the Moon,' where Jim Carrey infamously went method to portray Kaufman, and with Danny DeVito playing George.

In short, Shapiro had worked with the best of the best ... and leaves behind a storied career. He's survived by his lifelong partner and mother of his children, Melody, his kids and his many grandchildren. He was 91.