The visit marks the return of Kate to formal welcoming duties after she missed the Japanese emperor and his wife in June while she was undergoing preventive chemotherapy for cancer.
A royal salute and a carriage procession kick off a morning of pageantry on Tuesday before the emir heads to the Houses of Parliament to address both chambers.
A state banquet at Buckingham Palace will follow in the evening.
Britain is seeking deeper ties with the wealthy Gulf state, and Prime Minister Keir Starmer hopes to use the visit to secure "tangible benefits" for the country on security and the economy, his spokesperson said.
Starmer was elected in July on a promise to lift economic growth and is looking to potential wealthy investors such as Qatar to help fund his plans for new infrastructure and energy developments.
Qatar is already a big investor in Britain through the Qatar Investment Authority which owns the Canary Wharf business and entertainment district in east London, the Shard landmark skyscraper in central London, and stakes in blue-chip names such as Barclays and Heathrow Airport.
Accompanied by William and Kate, the emir and Sheikha Jawaher bint Hamad bin Suhaim Al Thani will arrive at Horse Guards Parade in central London to meet King Charles.
Queen Camilla will be absent from the outdoors part of the day as she continues to recover from a chest infection.
Charles, who continues his own treatment for cancer, and the emir will inspect the Guard of Honour at the famous royal parade ground and then travel to Buckingham Palace by carriage.
The two royal families, who know each other well as the emir has visited Britain on eight occasions between 2014 and 2023, will then be joined by Camilla for lunch at the palace.
The political side of the visit begins in the afternoon, when the emir heads to Westminster to address parliament. His formal meeting with Starmer is scheduled for Wednesday.
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