Found out that partner has been married twice..

Honestly,what he did long before he met you is nothing to do with you.I would never expect lists of GF's from my OH in fact I dont know any,its not something that was important.Its in the past.leave it there .Having said that I've stayed friends with several old BF's and a couple even worked for us ,it was never an issue with my husband.I married him ,the others are only friends

Thanks @NotSpaghetti that's how it seems to me now. At first it was a shock.
We have talked about it a lot, not so much the details of the marriage but more about the fact that it hadn't been mentioned before and also my reaction to it. I realised that a lot of my upset was connected to the fact that my ex was a compulsive liar and I mean that genuinely - not just that he sometimes lied to me - he lied to everyone about the most trivial things. So perhaps I am hyper-vigilant about being lied to.

I appreciate everyone's messages and thoughts. I feel now, after several days, that as others have said, it just doesn't appear in his day to day thinking. I'm confident that if I had asked him how many times he'd been married when we first met he would have told me the truth.

The OP never said it was hidden. Some replies sound as though it was a sort of secret or something deliberately not mentioned.
What the OP said was:

When I say 'found out' - he told me. We were talking about his ex-wife and it suddenly came in to my mind to ask if he's been married more than once. He told me as soon as I asked. When I asked why he had not told me before he said he never thinks about it, it was 40 years ago and a very short marriage.

He "never thinks about it" - this is a perfectly reasonable reason for it not to have come up. He told her about it with no qualms. It sounds matter-of-fact to me.

I think I wouldn't worry about it.

Reversing the genders, if a woman has a child that was adopted when she was young and forgets to mention it to a later partner. I do know several who have skated over their early years and two have actually asked me not to “go there”.

I think after a long relationship we should all be tolerant and understanding of any past mistakes that have not been revealed.

I think even if it isn't relevant to the person, it's only fair to be upfront about it.
A wedding was planned, booked, paid for, presumably after some sort of courtship.

There will have been in laws and families.
I can't understand airbrushing it out of your past.

My sister married at 18 the first time, me and another sister were bridesmaids, no children, unpleasant divorce. She then married again years later and had 2 sons.
I made her a collage of photos for her 40th not realising her 2nd husband and sons knew nothing of her 1st marriage. She hadn’t felt it was relevant.
I expect your partner feels the same.

I am with you Galaxy.
Somethings amiss here.
You cannot forget you were married just because it was short lived.

I have a friend who revealed to me unexpectedly she had been married three times, rather than the twice I had known about. She said the first marriage was when she was eighteen and only lasted 18 months with no children, so she didn't wish to consider as significant in the same way as her much longer marriages. She said that in those days ( the 1970s) ordinary people were still expected get married to be able to live together, unlike today when you can live together as a couple without being judged and without upsetting parents. I would have been shocked to find out but understand why it hasn't been mentioned. Would you expected him to have mentioned all other previous relationships? Perhaps subconsciously you had guessed, which is why you asked the question?
If the relationship is a good one, you will need to let it go.

I have recently been contacted by an old boyfriend from school he told me about his marriage but something didn't add up. He had moved away from our home town and his sister came in to where I worked to say he had got married and not to get into contact with him. He told me he was married 10 years after that date, and has never told me of his first marriage!!! His sister said she had gone to the wedding he said he had not spoken to her for years and she never went to his wedding. He might tell me one day perhaps he wants to forget it.

It is hard to imagine that the first marriage had been forgotten but easy to think that he had felt that admitting to two failed marriages might make him seem less desirable as a partner. Either way it was a long time ago......

I had a very good friend many years ago who I knew was divorced when I met her. She then married a mutual friend, and it was only at that point that I learned from her, that she had in fact been married twice before but the first marriage was a mistake and lasted less than a year. In the grand scheme of things it was on a par with other relationships and not what was is generally considered a marriage. She had put it out of her mind as it was meaningless. I don't see why you are feeling upset that your partner didn't tell you about this fleeting time in his life BGB31 and I wonder if during your life you've had relationships lasting maybe 9-12 months that you've not disclosed to him simply because it's never entered your head to do so. If you really like this person I think you'd be better not to make an issue out of it.

The number of people he loved enough to marry just doubled in your head. In his, his first marriage didn't even warrant a thought or a mention.

Just take some time to feel your way through and remember it changes nothing about how he feels for you

Don't take too much notice of those who tell you that you should know about it. 40 years is a long time ago and he probably doesn't think it was that important.

Personally I don't think that marriage is that important, unless perhaps if you're religious but also for legal issues.

I've been with my OH for 52 years. We lived together for 5 or 6 years before getting married. When we did, it was because back in the 60s people still made snide comments about me not being married so we got married.

To be honest I cannot remember the day or the year we got married but I do remember the date on which we met and if I celebrate anything, it's that date.

I agree MissAdventure a marriage is quite different from other past relationships. I don't see how it could be 'forgotten'.
I don't have a problem with the number of marriages, my husband was married twice before but the fact it was never mentioned.
However its water under the bridge now and your husband told you himself BGB so I would let it go.

I wouldn't worry about it. It was so long ago and has so little meaning to him now that he probably never thinks about it. We all have things that we don't tell. I don't ask DH about his past partners and he doesn't ask me about my Jezebel period. Our relationship isn't based on who we were then but who we are now.

It's not every detail.
I would never divulge how many people I've slept with, but I think I would remember if I'd married one of them.

I know two people with early (young) disastrous marriages that are virtually obliterated from memory. I think I'd ignore it.
Obviously not partners but one, a very close dear friend who is always open and genuine and who I'd loved for years told me 25 years on, when something sort of "triggered" it.
She had no photos, no mementos, no belongings. It was not a secret. Just an absence.

I differ with many other posters.

Even if the marriage was a short one and a long time ago, I would still have liked to be told about it earlier.

All this ‘forgetting’ something as important as a marriage seems odd to me.

I'm sorry but why is every detail of someone's life before you even knew them any of your business. It doesn't affect you. I don't understand at all. Have you been into detail of every man/person you've slept with.

Just because it means little to him doesn't mean the op should respond the same way, though.
It's not all about one person in a relationship.

If it means so little to him, why bother about it?
We all have secrets to keep that even our nearest and dearest don't know.
Unless this first marriage seems to be affecting him in any way - and it sounds as if it isn't- put it back in the box and get on with your life together

He didn't lie to you, you love him, let it go!

I think. if I ever met a new bloke and we decided it was going to be serious, I would want a tit for tat open conversation about all the issues we both thought were important to get everything out in the open up front, so there were no hidden secrets or surprises likely to emerge down the line. Number of marriages would certainly be on that list.

I'd be a bit wary in case he had forgotten anything else.
I think it should be fairly relevant, making vows to someone.

Women are allowed to have whatever boundaries they wish. I would find it odd that he hadnt mentioned it.