Speaking ahead of the register's launch, Alison White conceded a lot of lobbyists would not be listed on it.
But she said her job was to make sure the rules were implemented and she would do so with "extreme verve and vigour."
But one critic of the register said it was "fake" and full of "loopholes".
Lobbyists are companies or individuals paid to influence government decisions.
The coalition promised to introduce a statutory register of lobbyists when it came to power in 2010, and legislation was passed last year, with ministers saying the move would help clean up politics.
However, the new register will only cover consultant lobbyists and will not apply to lobbyists working "in-house" for companies or organisations.
A person or company will only need to be registered if they are contacting a government minister or permanent secretary in a government department - leading to criticism it will not come close to reflecting the scale or nature of lobbying that goes on across Whitehall.
But Ms White, who is independent of the government, said her task was to implement the law as it stood not to question the powers she had been given.
"We are having something that has never been in place before," she told BBC Radio 4's Today programme.
"This is a statutory register. The industry tells me it is a professional industry. I expect them to conform with the legislation. It is my job to make sure they do so."
Ms White, who was appointed as the registrar of consultant lobbyists in September, acknowledged the register would not indicate different "levels of lobbying".
"The legislation was quite narrowly drafted to address a particular issue, which was that it was not always particularly clear, when ministers were being lobbied, who the client of the lobbyist was. That is what the register was set up to do," she said.
She added: "This will give anyone who wishes to access the register the opportunity to be able to see who is lobbying and being paid for it and who their clients are. That is the main information the register will contain.
"If a lobbyist sets out to influence government policy in some way, they would have to be registered. If they are not registered, then they would not be conforming. They have to go on the register before they lobby."
Ms White said last month that the register would not have any entries on its launch day but she had received more than "20 expressions of interest" about joining and companies and individuals might be required to sign up retrospectively.
Potential registrants will be able to log on and create an account to file an "information return".
But Tamasin Cave, from the Alliance for Lobbying Transparency coalition, said "hardly anyone" in the industry would be covered by the provisions of the new law.
"You have a huge commercial lobbying industry, and very few of them will have to sign up," she told Today.
"The second flaw is that it won't show the interaction between lobbyists and government, and that is what a register is for.
"It is to show the activity - who is discussing what with whom, but this shows nothing of this interaction. It is a fake register."
The woman in charge of the government's new register of lobbyists has said she expects the industry to "conform" with the statutory system.