Which bands/artists opened the prog portal?

Mogwai is another gateway band that launched me into a more recent favourite of mine, Post-Rock.  It's funny, I've never really realized it, but I was into Mogwai way before I ever got into Prog, as a full genre. I'm a huge Smashing Pumpkins and Sonic Youth fan, have been since a very young age, and Mogwai certainly took inspiration from those two artists in terms of how they create their sonic palette and their choice of instrumentation and effects. At the time, I wanted to discover more artists like those two bands and I came across Mogwai. I had no idea they were dubbed this so called "Post-Rock", I didn't even know that the term existed, all I knew was that it was certainly more unconventional and atmospheric than anything I had heard before that, and I loved it. I went out and bought Young Team the next day, and I was just in awe - immersed in the atmospheres. 

   When I was a teenager I listened to Pink Floyd, Jethro Tull and Barclay James Harvest among many other bands, but I had no idea what is Prog. So as a first band it must have been BJH. But the band that opened the door to Progressive for real was Eloy. Power and the Passion first, Dawn second, Ocean third, and the rest is history...
 Later on I discovered that many of the bands I use to listen are falling into the Progressive Rock category.

Probably hearing Kansas's "The Pinnacle", I'm so glad that was my first ever prog song!
You could imagine my face being 14 and only ever hearing pop music my whole life, then listening to The Pinnacle.
I wish I could listen to it for the first time all over again.

In the School summer holidays of 1979, when I was fifteen, I bought 4 albums: And Then There Were Three,  Selling England by the Pound, The Dark Side of the Moon and Close to the Edge (following the advice of the guy who used to work in my local record store).  the gate was not so much opened, as ripped off its hinges.  A lifelong passion for music (prog in particular) ensued.

 ^ I believe it was Gilmour who said he never thought of Floyd as 'Prog'--  Artrock maybe but not Prog

Back in the 70s I was listening to Pink Floyd, Jean Michel Jarre and some prog singles from Tull and ELP

Originally posted by Green Shield Stamp 'Prog' is very much a retrospective term.  I don't think any of the classic prog bands (Pink Floyd, Genesis, Yes, Jethro Tull, ELP, King Crimson etc) considered themselves to be prog, or were labelled as such at the time.   This lumping together of bands under the prog label occurred quite late on.  I'm not sure when, but I think it was probably in the late 80s.  Interestingly, In interviews members of Genesis seemed to be quite horrified to be put in the same category as bands like ELP, they saw themselves much more as a song-based band.   Highly creative bands (such as Pink Floyd) tend to see themselves as doing their own thing, playing the music that best expresses themselves and their unique musical vision. Journalistic categorisation and compartmentalisation runs counter to what these sorts of bands are about.   Labels are useful but they are also very limiting as they set pre-conceptions and narrow definitions.  I suppose the most complimentary label that can be attached to highly creative and original bands is 'uncategorisable'.  So, it is unsurprising that many of the great 70s bands balk at the term 'prog'.

True.
And since I was young during that era, it's only in retrospect that I can label my favorite music then as "prog". But nevertheless, I'd say The Beatles,The Moody Blues and Pink Floyd. They all came before King Crimson and were an inspiration to them. Crimson even hired The Moodies producer for their first album. It didn't work out though, but the sound is clearly that of of the Moodies. The only innovative track on In the Court... is Schizoid Man.