Yhyjyk

I am currently trying to obtain a German passport, my mother is German and therefore I have a right to citizenship. However, German citizenship laws say that you can lose German citizenship if you apply and choose to take up another passport.

I obtained a U.S. passport while I was a minor, because my father held a U.S. passport and I was a legal U.S. resident, therefore I was automatically eligible as his child.

I have spoken with the German embassy in London (I am also British, by birth), and provided all relevant documents and they confirmed with me my eligibility for a German passport, however they require proof of how/when I obtained my U.S. passport to prove that it does not contravene the aforementioned policy about obtaining new citizenship.

That's where I'm having some trouble, I need documentation to prove that I obtained citizenship while still a minor. I became a U.S. citizen quite close to my 18th birthday so my passport issue date fell after my 18th birthday. I don't have any documentation from the process of getting my U.S. citizenship, so I'm at a loss of how to get the documents I need.

I looked up the process on the USCIS website, and it looks like a form N-600 would have been submitted. Is there some way I could get a copy of it from the government, or is there another document that would be able to provide the proof I need?

Update:
If there is a way to prove when I received my green card, that might also suffice. As with that information I could prove that on that date I fulfilled all of the criteria to be automatically a U.S. citizen.

and it looks like a form N-600 would have been submitted

Not necessarily. If you were a permanent resident under 18 living in the US with a US citizen parent, you automatically became a US citizen, and thereafter you can apply for a US passport directly without getting a Certificate of Citizenship first. Many people who derived US citizenship as permanent resident minors just get US passports and never bother to shell out the $1,170 fee to file N-600 for a Certificate of Citizenship.

If you could prove that you were a US permanent resident before you turned 18 (e.g. a copy of your old green card, or I-485 approval, or entry stamp on an immigrant visa) and met all the conditions for automatically deriving US citizenship, that could technically prove that you became a US citizen before 18, but I am not sure whether German authorities would be willing to basically adjudicate a complicated area of US nationality law, so I am not sure they would accept such evidence.

If all else fails, you may have to file N-600 to apply for a Certificate of Citizenship (paying the huge fee) now if you had never gotten one (or get a replacement by filing N-565 if you had gotten one) because that is the one thing that will definitively state when you became a US citizen.

Here is everything that I learnt throughout this process:

I was not born a U.S. citizen, and I did not naturalise. Instead, I inherited U.S. citizenship at the moment I became an LPR (Legal Permanent Resident), because I both had a parent with citizenship and I was under the age of 18.

There is no paperwork to file to inherit citizenship this way, it is completely automatic and as a result there are people in the U.S. that do not know they are citizens. In order to assert that I was now a U.S. citizen, I just had to apply for either a U.S. passport, or a Certificate of Citizenship. There is no deadline to do this, because I was a fully-fledged citizen from the day of approval of my Permanent Residence application.

When I applied for my U.S. passport, I simply had to prove that I was a child at the time of receiving my Green Card, and that one parent was a U.S. citizen.

This is exactly what I had to provide the German consulate. I provided my own birth certificate, my I-485 form (with approval date) and my father's naturalisation certificate. Those were accepted without question.

To obtain a copy of my Form I-485, I had to file a FOIA request--I used a Form G-639 to make it easier--and I emailed it to uscis.foia@uscis.dhs.gov.

I did attempt to get a copy of my passport application, as I filed that while under 18, but I was told by the State Department that they did not have a copy of it. This was not a surprise as when I applied for a passport at the age of 17, I was told by the official that they do not keep a record of my application, so I should also have a Passport Card in case my passport goes missing and I must prove citizenship.

Under German citizenship law, one loses citizenship when "voluntarily" obtaining another citizenship. The reason that I did not lose my citizenship when I became a U.S. citizen was because: I was a child, and it happened automatically. Therefore, it is not considered voluntary. Luckily, I did not have to explain this, since it was a consulate in the U.S. (Houston, TX) that I visited, they were well aware of this. I had some trouble with the London embassy not understanding all of the U.S. citizenship qualifiers.

I hope this helps anyone else that finds themselves in a similar situation! I was completely lost when I started this process, but three months later I have my German passport in my hand.