Thee is evidence that the use of episode titles was normal in science fiction television programs made in the USA in as early as 1949. But I note four problems that make assuming that is correct a little uncertain.
Others have mentioned Flash Gordon (1954) and Rocky Jones, Space Ranger (1954) as the earliest science fiction television programs with separate titles for episodes.
The IMDB list of Flash Gordon (1954) episodes and Wikipedia both list "Flash Gordon and the Planet of Death" as the first episode on October 1, 1954.
According to the IMDB list of Rocky Jones, Space Ranger episodes, the first episode on 10 April 1954 was part of a three episode story and had the title of: "Beyond the Curtain of Space: Chapter 1". But chapter 2 of that story was supposedly on 2 Mar. 1952, which seems out of order. The first single episode story was broadcast on 1 May 1954 with the title of: "Escape into Space".
But the different stories are not listed in chronological order in this list. In the list in Wikipedia the stories seem to be listed in chronological order, but some have different dates. "Beyond the Curtain of Space" first episode is dated to February 23, 1954, and "Escape into Space" is dated to April 27, 1954.
Tales of Tomorrow is an American anthology science fiction series that was performed and broadcast live on ABC from 1951 to 1953. The series covered such stories as Frankenstein, starring Lon Chaney, Jr., 20,000 Leagues Under the Sea starring Thomas Mitchell as Captain Nemo, and many others.
The IMDB episode list for Tales of Tomorrow lists the first episode as having the title: "Verdict from Space" and airing on 3 August 1951:
However, the IMDB entry is not absolute proof that the episode title was used back in 1951 so I suppose that some doubt remains.
And Tales of Tomorrow was an anthology series where each and every episode was a totally separate story with different characters and setting, and so some people might claim that an anthology series would be much more likely to have separate episode titles than a series with regular continuing characters.
Tom Corbett: Space Cadet was first broadcast on October 2, 1950. The earliest episode title in the IMDB list is "Trial in Space" 21 July 1951.
Space Patrol was a television series with recurring characters that was broadcast from 1950 to 1955. Wikipedia says that it was first broadcast 5 days a week starting March 9, 1950, while the IMDB episode lists gives the first episode as "Treachery on Mars" 30 December 1950.
Captain Video and his Video Rangers was broadcast between June 27, 1949 and April 1, 1955.
In the IMDB episode list, the earliest episode with a complete date is "Misc. Vid 4; Su Ching Sing" 1 June 1950; the earliest episode with a month and year is "Lazy K Ranch" Apr. 1950; and the earliest episode dated only to a year is "Captain Video Prepares to Visit Regas" dated 1949.
So a quick search among early science fiction television series in the United States of America gives the impression that it was normal and usual for episodes to have titles in the early 1950s and even in 1949.
However, Thee are four problems that add uncertainty.
I noted that the sources do not give episode titles for every single episode of some of those series.
Of course it is impossible to be certain that any episodes actually had titles when they were first broadcast without more research such as going to websites dedicated to specific television series, watching episodes online and noting if they have titles that seem to be original, and so on.
I also note that there might have been some some early American science fiction television programs with or without episode titles that I failed to check on.
I also note that there may have been a number of early science fiction programs produced in other countries in the 1950s and 1940s, with or without episode titles.
Added 01-28-2020. Comment by Klaus AE Mogensen say that episodes of Space Patrol, Tom Corbett, and Captain Video on YouTube don't have episode titles while episodes of Tales of Tomorrow do have titles.
Barrowc's answer shows a image of the title card with episode title of the first episode of The Quatermass Experiment, "Contact Has Been Established", broadcast in the UK on 18 July 1953.
Therefor it seems that the earliest examples known so far of science fiction television programs with individual episode titles are Tales of Tomorrow, an anthology series in the USA in 1951, and The Quatermass Experiment in the UK in 1953.
Some people might claim that Tales of Tomorrow doesn't count because it is an anthology series instead of a series with continuing characters, and other people might claim that The Quatermass Experiment doesn't count because it was a limited series of six episodes. Picky, picky. And there is always the possibility that some earlier science fiction television programs did have individual episode titles.