The Man From UNCLE was so successful in his second season (1966) that the producers decided to create a parallel series (or spinoff) using the same scenario of the original. And by the same boss, Mr. Alexander Waverly (Leo G. Carroll).
The Girl From UNCLE was a pioneer in series starring women-spies (as Alias and Homeland). Unfortunately it came in 1967, during the third season of The ManFrom UNCLE. And the producers of the two parallel series wanted to ride the wave of parody that made the success of Batman. It was a wrong choice for the many fans of the series. The Girl From UNCLE lasted only one season and The Man would be canceled in early 1968.
The Girl From UNCLE starred the agent April Dancer (Stephanie Powers). She was baptized by Ian Fleming, the creator of James Bond. April had as a partner the british Mark Slater (Noel Harrison). The series was never a hit, like the original. Still watch The Girl From UNCLE today (on DVD) is a fun time travel.