He began acting professionally in the mid-1960s and quickly rose to prominence for his supporting role as Ensign Pavel Chekov in Star Trek: The Original Series (1967–1969).

After a professor encouraged Koenig to become an actor, he attended Neighborhood Playhouse School of the Theatre in New York City with fellow students Dabney Coleman, Christopher Lloyd, and James Caan.

John Delwyn, who is recommended for Officer Candidates School by the series protagonist, Lt William T. (Tiberius) Rice, played by Gary Lockwood; (in episode 27, "Mother Enemy", aired on April 4, 1964).

The plot twist, at the height of the US–Soviet Cold War, is that Sgt Delwyn's visiting mother is a prominent, and politically active, American Communist Party member.

[citation needed] In 1964, Koenig portrayed a New York City juvenile gang leader in an adaptation of Memos from Purgatory for The Alfred Hitchcock Hour.

The studio's publicity department, however, falsely ascribed the inclusion of Chekov to an article in Pravda that complained about the lack of Russians in Star Trek.

[6][11] When the early episodes for season two of Star Trek were filmed, George Takei was absent while completing the movie The Green Berets, so Chekov was joined at the Enterprise helm by different characters.

The show's producers had declined to hire Koenig for budgetary reasons and the actor wasn't even aware that he would not be included in the cast until a fan informed him at a Star Trek convention.

He was a "Special Guest Star" in twelve episodes and, at the end of the third season, the production company applied for an Emmy nomination on his behalf.

He has written several books, including Warped Factors: A Neurotic's Guide to the Universe (an autobiography), Chekov's Enterprise (a journal kept during the filming of Star Trek: The Motion Picture) and Buck Alice and the Actor-Robot (a science fiction novel), which was re-released in 2006.

[22] Koenig has taught classes in acting and directing at UCLA, the Sherwood Oaks Experimental Film College, the Actor's Alley Repertory Company in Los Angeles, and the California School of Professional Psychology at Alliant International University.

[23] In 1987, Koenig directed his original one-act play The Secret Life of Lily Langtree at the Theatre of NOTE in Los Angeles.

[24] In 1997, Koenig starred in Drawing Down the Moon, an independent film about a Wiccan woman who attempts to open a homeless shelter in a small Pennsylvania town.

Koenig played Joe Merchant, a local crime lord obsessed with chaos theory who sends his thugs to intimidate her into shutting down the shelter.

[25] In 2004, Koenig co-starred in Mad Cowgirl, an independent movie about a meat-packing health inspector dying from a brain disorder, in which he played televangelist Pastor Dylan.

[27] In 2013, Koenig ventured into the steampunk genre, starring in the short film Cowboys & Engines alongside Malcolm McDowell and Richard Hatch.

In 2017, Koenig appeared in the 1980s throwback Neil Stryker and the Tyrant of Time from director Rob Taylor, battling puppet goblins as science officer Ray Nabroski.