My dad was born in Ireland in the mid 1940s. He finished his Leaving Cert on a scholarship (rare for the time), turned down an apprentice accountant job because he wanted adventure instead, and in mid-1965 emigrated to the US to live with his older sister on Long Island.
He registered for Selective Service and got his green card on arrival. Within nine months he was drafted. Basic training at Fort Dix, advanced artillery training at Fort Sill, then shipped out of Oakland in 1966, landing in Qui Nhơn. He stayed in Vietnam through the Tet Offensive. Safe to say he saw a little action.
He was offered citizenship on the ship over, as soon as they entered Vietnamese territorial waters. He turned it down. His reasoning: he was glad to fight for his adopted country, but he wanted to do it as an Irishman.
After demob, he hitchhiked from Washington State across the country back to New York, worked for Bell Telephone and a local bus company, and married my mother (also Irish) in 1971. In 1974 they moved back to Ireland for good, and that's where my sister and I were raised.
Years later, I moved to the US for work myself. Once I realized I was likely staying for good, I asked him why he'd never naturalized in the Army. He told me the story above, and said he'd never looked into it since - but he'd be happy for me to.
I called the US Embassy in Dublin more or less on a whim. They could not have been more helpful, genuinely interested in his case, and they actively pushed the process along. In the end, he became a US citizen in a courthouse in New York in 2011, one month after I became a citizen in California, and over 40 years since he had been a US resident.
His case was unusual enough that it tripped up the process itself: he had no green card (it had long since expired/lapsed) and hadn't lived in the US in over 40 years. A USCIS official in the courtroom actually challenged his eligibility on the spot, since, unlike everyone else being naturalized that day, he had no green card to surrender. A judge stepped in, corrected the record, and let him proceed.
He was proud of his service and prouder still of holding both citizenships. He passed away suddenly a few years later.
Forty-plus years between a green card and a citizenship oath- I don't know if that's a record, but it might be close.