Morse's death was confirmed Wednesday, though his cause of death is currently unknown.

The actor's work on "Mad Men," playing Bertram Cooper for 74 episodes, got him 5 Emmy nominations over its 8 years on the air. Bertram, or Bert, was one of the founding partners of the Sterling Cooper Advertising Agency, where the show takes place.

Always dressed with a bow tie, Bert reluctantly agreed to a merger with British agency Puttnam, Powell, and Lowe ... he would then go on to be a founding partner of Sterling Cooper Draper Pryce.

The character would die the night after watching the televised Apollo 11 moon landing ... Bert appeared after his passing in one of Don Draper's visions, played by Jon Hamm.

The Tony and Emmy-award-winning actor left behind a career on both stage and screen, steadily working for 6 decades. He got his start on Broadway in the '60s with a production of "How to Succeed in Business Without Really Trying" ... his performance snagged him a Tony for Best Actor in a Musical.

He went on to win another best actor Tony award as Truman Capote in the 1990 play "Tru" ... for which he got an Emmy for Leading Actor in a Limited Series.

After "Mad Men," Morse came back to Broadway, working with Nathan Lane in the 2016 revival of "The Front Page."

Tributes have been pouring, including one from Larry Karaszewski, the VP on the board of governors of the Academy of Motion Picture Arts and Sciences, who called Morse "a huge talent and a beautiful spirit."

Broadway composer David Krane also paid tribute, saying "He had those cool, clear eyes of a seeker of wisdom and truth. Wishing that eternal boy eternal peace in his new forever home. He succeeded without really trying."

Morse is survived by his wife Elizabeth Roberts and his 4 children -- Charlie, Robin, Hilary, and Andrea.

He was 90.