Agarwal, 39, grew up in humble surroundings in India and left for London toset up foreign exchange giant RationalFX, which had a turnover of over 1.3 billion pounds last year.
The businessman who has also founded international money transfer service Xendpay is featured in the 'Sunday Times' rich list with a fortune of 90 million pounds.
"The mayor and I are determined to build a coalition that ensures the needs of business and financial services are at the fore over the coming months of negotiations with the EU. My first priority will be to listen and engage with businesses, to hear their concerns and deliver reassurance," said Agarwal, who will step down from his businesses to focus on his new role at City Hall in London.
"This openness to talent and enterprise must not change as a result of the referendum. I share Sadiq's belief that a thriving economy is critical to ensure that all Londoners can share in our great city's future success."
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Agrawal was Khan's business adviser during his mayoral campaign.
Khan, the son of a Pakistani bus driver, took charge as London's first Muslim mayor last month.
"I know that Rajesh is the best person for the job of protecting jobs and growth in London as we deal with the fallout of the referendum," said Khan.
(REOEPNS FGN 28)
Cameron's resignation list had proved hugely
controversial after the names he had recommended were leaked to the media earlier this month.
Henominated a number of his political allies for peerages, including Indian-origin Conservative party donor JiteshGadhia, a former senior managing director at US private equity firm Blackstone.
An outgoing British prime minister has the right to draw up aresignationhonoursliston leaving office.
Cameron'slist, which includeshonourssuch as knighthoods and OBEs, was leaked to 'The Sunday Times' and caused an uproar as it indicated that he had sought to reward Remain campaigners in June's European Union referendum, his Downing Street staff and donors, two of his former drivers as well as his wife Samantha's special adviser cum stylist.