Simon Kirby told a parliamentary hearing that the Treasury is leading the charge for the sector as Theresa May's Cabinet thrashes out the Government's negotiating stance.
He said: "The Treasury is responsible for financial services.
"He (Mr Hammond) has made that case loud and clear at Cabinet. All departments are working together to get the best possible deal."
Kirby added that he and the Chancellor had met financial service representatives and other stakeholders to sound out their views.
His comments come amid speculation that Mr Hammond threatened to quit his role after attempting to put the brakes on the Government's "hard Brexit" plans - rumours the Treasury has denied.
Fears in financial markets over a hard Brexit have grown recently, best illustrated in the sell-off in sterling, as it would see the UK ditch the EU's single market, a move that many argue would be catastrophic for businesses and the economy.
A hard Brexit would also almost certainly see businesses in the Square Mile lose passporting rights, which allow them to do business in Europe, dealing a hammer blow to the City.
Kirby added: "It's fair to say that even in banking - you would think banks are all concerned about passporting - there are differing opinions, with challenger banks to big banks.
"For big American banks, access is important. If you talk to the insurance industry, there's a spectrum of opinions.
"It's something that's very important to some, less important to others. For a lot of domestic-facing financial services companies, it's not at all important."