What sitcom star helped get Star Trek made?

There are so many shows in the Star Trek franchise that the original was officially renamed "Star Trek: The Original Series" (hitherto referred to simply as TOS).

Yet those first three seasons didn’t go well, and "Star Trek" was canceled. It wasn’t until TOS was picked up and re-aired across the world in syndication that Gene Roddenberry’s vision was eagerly embraced by adoring fans. The rest, as they say, is history.

Perhaps no other entertainment franchise owes as much of a debt of gratitude to syndication as this one, which now includes 11 television series (and counting), 13 feature films, and countless books, video games, conventions, and so on. 

But the show very nearly never went beyond a single pilot. And if it weren’t for a particular sitcom star, TOS probably never would have been made.

Who helped this show boldly go where no one had gone before?