The very-first end-credit scene was apparently from The Silencers movie which was released in the year 1966 based on this article

The first modern post-credits scene seems to be the tag at the end of The Silencers, a 1966 James Bond spoof starring Dean Martin, according to the site What's After the Credits? The scene in question parodied the familiar James Bond tradition of teasing a future installment, but rather than the simple text card of "James Bond Will Return" seen in the 007 films, The Silencers featured actual footage of Martin's Matt Helm lounging on a bed with several scantily-clad women while overlain text reads: "Coming up next: Matt Helm meets Lovely Kravezit in Murderer's Row." Helm himself then says, "Oh my God," and hides his head in his hands.

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The end-credit scene which became popular and spawned various future end-credit scenes is from the Muppet movie (1979)

It wasn't until 1979, after The Muppet Movie featured Animal telling the audience to "go home" after the credits finished rolling, that post-credits scenes began to appear somewhat regularly — and almost exclusively — in comedies.

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While the Muppet movie popularized the concept of "comedy" end-credit scenes which is just for relief and an extra addition to the movie's comedy, the end-credits of Marvel movies variety which teases the upcoming movies was introduced in the movie Masters of the Universe (1987). But due to it's collections and general reception, it did not pick steam...

After the credits roll in 1987's Masters of the Universe, the presumed-dead Skeletor rises from under water to proclaim that he'll be back, but the film's poor box office performance prevented that from ever happening.

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TLDR:

• The Silencers introduced end-credits
• The Muppets movie popularized it and introduced the "comedy" end-credits which has no significance to the plot or tease the other movies
• Masters of the Universe introduced the "tease-hook for other movies" end-credits...

The Muppet Movie (1979) was the first movie with a post-credit scene according to this Wiki page which lists all films with post-credit scenes.

The Muppet Movie (1979) is obviously the first traceable true post credit scene instance but the older is 1963 with the James Bond film From Russia with Love, which was the first Bond film to show the ubiquitous "James Bond will return in..." at the end of the credits. (Source :Wikipedia)

As for TV:

Several episodes of Monty Python's Flying Circus (1969–1974) have scenes after the ending credits. For example, The Money Programme (first aired 1972-11-02) has ending titles, then "Six more minutes of Monty Python's Flying Circus", followed by the Argument Clinic sketch. The episode ends after an announcement of "One more minute of Monty Python's Flying Circus".

In the episode Michael Ellis, first aired 1974-11-07, the ending credits immediately follow the opening credits, which means that essentially the entire episode is after the ending credits. The episode ends abruptly after a shop assistant in the "Different endings" sketch asks Well, how about a sudden ending?.