Visas, applications and tests can add up to thousands of pounds.
Kensington Palace says Ms Markle will be treated like any other applicant. So how difficult is it for the average American?
Freddie O’Farrell and his American partner Bethany Birdsall are right at the start of the process and applied for a fiancée visa just as news of Prince Harry and Ms Markle’s engagement broke. It’s a process, they say, that is already proving stressful.
Forced to return home to Minnesota to submit the application, which takes three months to be considered, Ms Birdsall is prohibited from working in either country during this time.
Mr O’Farrell, 24, said: “She’s basically killing time until she can come back and then once she is in the UK she can’t work, or apparently even volunteer, until we after get married and she has a marriage visa.
“It seems crazy to me that she can’t work, and has to fly home and twiddle her thumbs for three months, if not longer, whilst we wait for an outcome.”
That puts extra pressure on finances – on top of around £3,000 needed for both a fiancée and marriage visa.
“Because we have no dependants, we are quite a simple case,” he said. “But even for us we are looking at spending a lot of money and being apart for three or four months.
“It’s definitely a strain on any relationship, but it’s much harder for her because she can’t work when she gets here, so it’s a lot of waiting around.”
Ms Birdsall has previously worked with children of undocumented migrants in Chicago and, when she is legally allowed to work, wants to do a similar job in the UK.
Mr O’Farrell added: “Frankly, the system is very difficult to navigate, prohibitively expensive – though I imagine that is probably on purpose – and very slow.”
‘Confusing and frustrating’ system
American writer Jessica Pan, from Texas, recognises Ms Birdsall’s fledgling struggles as part of a “confusing and frustrating” system that is constantly changing.
She moved to London to be with her now-husband following a period working in China and dating long-distance.
She originally planned to apply as a skilled migrant.
“I had a Master’s degree and made enough money in China to qualify for it. I planned my life around this,” she remembers.
But rule changes threw this plan into chaos; suddenly her Chinese salary no longer met the new requirements.
As a result, she applied for a fiancée visa and moved to London in 2012, re-applying after two years as required, and finally becoming a British citizen in 2016.
“People plan their entire lives around these rules that change at the drop of a hat,” she said.
This instability drove Mrs Pan to apply for citizenship as she “wanted to see an end to that constant feeling of the unknown”.
Tests and paperwork
Applying for citizenship costs £1,282, and applicants must have spent no more than 270 days outside the UK in the past three years.
Karen Marshall, 61, who became a British citizen in 2013 after living in the UK and being married to a Briton since 1986, described the intricacies of the test as “a challenge from start to finish”.
She failed her first attempt, not over the test itself, but rather a mistake in the initial paperwork.
“I accidently ticked biometric residential permit on the online form, not realising the difference between my normal residential permit,” she said.
Mrs Marshall, a retired piano teacher, also found the questions “somewhat ambiguous”.
“One asked ‘which group is better off now than in the 1920s?’. I think they wanted the answer to be women, but everyone now is better off now than in the 1920s,” she added.
Applicants can request for their test to be remarked and will be given feedback on the number of questions answered incorrectly across each area, although they do not find out exactly which questions they got wrong.
Beyond the test, the practical challenge of collating all the paperwork is another difficulty for applicants, who must provide detailed travel information in and out of the UK over the past five years.
‘People take me more seriously’
But despite the struggles, Americans see many advantages to gaining British citizenship.
For Mrs Marshall the “fantastic” NHS proved a key factor.
Mrs Pan feels she is “taken more seriously,” through her new-found status, especially after gaining the right to vote.
“I became eligible right before the EU Referendum. It was important to me that I got to have a say and vote Remain.”
But despite the result, Mrs Pan says the greatest strength of citizenship is “knowing you have a say”.