TV Century 21, as it was originally called, launched on Wednesday 20th January 1965 with this bold logo and its distinctive newspaper-style cover...
A year later with issue No.52 (January 15th 1966), the comic had its first revamp to celebrate the arrival of the Thunderbirds strip to its pages. The new box-style logo was more flexible for the cover designers, able to be placed in the corner of the page, or under a headline (as shown), or even on a few occasions, tilted 90 degrees to one side.
That design continued for quite a while, until issue No.141 (September 30th 1967), when it was redesigned as a banner again, now incorporating the Spectrum logo. Captain Scarlet had arrived in TV21 in a big way!
A few weeks later, with issue 155 (January 5th 1968), came huge changes for the comic. The title was simplified to TV21 (which everyone had always called it anyway), the logo was more basic, changing its colour and background colour every week, and most dramatically, the unique newspaper-style covers were replaced by a regular Captain Scarlet strip. (This cover illustrated by Ron Embleton.) Presumably sales had dipped and this was an attempt to show potential readers that TV21 was a comic, not a magazine, but in doing so it lost some of its character I think.
The covers were very dynamic though, particularly this one by Mike Noble. Issue 162 (Feb 24th 1968) was the first to show how adaptable the new logo could be; one week it could be a banner, the next week it could be restructured like this, to suit the cover image.
Several months later, a merger gave us TV21 and TV Tornado with issue 192 (September 21st 1968). The comic was moving even further away from its original concept, with more non-Anderson material (Department S, The Saint, etc) and free football cards! Cover art by Frank Bellamy though, so that's something.
Drastic steps to regain the comic's identity were implemented with issue 218 (March 22nd 1969), as a new logo and newspaper design hearkened back to the early styles.
That first series of TV21 ended with issue 242, - or it did until last year, when an official all-new TV21 No.243 was produced as an extra in Network's Supermarionation Blu-Ray boxset (see here). Editor Martin Cater gave us a new logo in a grand TV21 tradition.
The second series of TV21 began with a couple of weeks delay due to strike action but in September 1969, TV21 and Joe 90 No.1 appeared. New logo, content heavily influenced by its merger with Joe 90, and bloomin' football on the cover!
With the third issue, the blue background was replaced with a regular purple one...
...and with issue 4 (18th October 1969), the football photos were thankfully dropped in favour of cover paintings highlighting one of the stories inside...
Issue 36 saw the end of the Joe 90 strip so TV21 No.37 (6th June 1970) had another new logo...
It seems the editor was still deciding on a colour for the logo, as the following week saw 'TV' in red...
...but with issue 39 (20th June 1970) the new logo was tweaked and finalised, just in time for new strips to begin. The new strips had nothing to do with television programmes, perhaps to avoid paying fees to TV companies.
With issue 42 (11th July 1970), the cover paintings were replaced by the Star Trek strip starting on the cover. (Star Trek had joined the comic from Joe 90 when the two titles had merged.)This one by Mike Noble, although other artists such as Ron Turner and Harold Johns also had stints on the strip.
TV21's cover format then stayed the same until the final issue of its "New Series" (No.105). The week after, it merged into Valiant dated 2nd October 1971, with the only strips passing over being Star Trek and non-TV strip The Tuffs of Terror Island. A sad decline for a great comic, but it had a good run!
(All of the covers shown here are scanned from my collection, except for new series No.42 which is from an eBay page.)