Bill Crothers has come under fire for working at the now-failed finance firm while still on the government payroll.
It has now come to light he took on another trustee role without seeking the advice of the necessary committee.
Mr Crothers apologised to the Advisory Committee on Business Appointments for what he called an "honest mistake".
But the chair of the committee, Lord Eric Pickles, said he had breached government rules.
The news comes after Lord Pickles warned the existing guidelines on ministers and top officials taking jobs with private firms needed urgent reforms as there did not appear to be "any boundaries at all" between civil servants and the private sector.
Labour says things are becoming "murkier" and lobbyists have an "open door" to government.
The government has announced a review of contacts between top officials and ministers after the row over Greensill Capital, access, influence and lobbying has engulfed Westminster.
The review will include former prime minister David Cameron's dealing with the firm, as well as Mr Crothers.
It will be led by Nigel Boardman, a non-executive board member of the department for business.
A number of Commons Select Committees have also announced their own inquiries.
On Wednesday, Mr Johnson ordered Conservative MPs to vote down Labour's call for a "full" MP-led inquiry into Greensill and lobbying more widely.
It emerged on Tuesday that Mr Crothers - a former government procurement officer - joined Greensill as an adviser in 2015 while still working as a civil servant, after getting the go-ahead from the Cabinet Office.
But an exchange of letters between him and Lord Pickles, published on Thursday, also revealed he had failed to alert the committee about a different job he took in 2016 as a trustee with the Chartered Institute of Procurement and Supply
Mr Crothers apologised in his letter for what he described an "honest mistake", saying he did not think he had to report the role as it was unpaid and with a not-for-profit charity.
But Lord Pickles said the rules applied to both paid and unpaid jobs and it was therefore a breach of the regulations.
Earlier, the Tory peer Lord Pickles told the Commons public administration committee he had warned of a scandal but the Greensill lobbying affair was "not where I expected it to come from".
He added: "There is nothing wrong with lobbyists. What is wrong is with unregulated lobbying, secret lobbying - people getting an undue advantage.
"Our entire political careers have been built by lobbying of some sort. But where it becomes wrong is where it is not properly regulated and is transparent."
Speaking about Mr Crothers, Lord Pickles said it was not unusual for civil servants to have second jobs and to share their expertise.
But he said the former civil servant's "excuse" when justifying his work with Greensill surprised him.
"It did seem, talking to other colleagues who had enjoyed being civil servants in the past, that they also shared my surprise - this seems to be a new thing, or a new excuse," he said.
He said contractors and consultants to the government should have to sign a memorandum of understanding about the restrictions that would be placed on them after completing their public sector work.
Lord Pickles also expressed his frustration that his committee had a "very limited and defined role", but added that most people do follow the rules and it is "only a tiny minority" who cause problems.
Speaking on a visit to Dartmouth College in Devon, Boris Johnson said he agreed with Lord Pickles that reform was needed.
The PM said: "I think the most important thing is for us to get to the bottom of it properly and I want all ministers and civil servants to be making the information that needs to be known known to Mr Boardman and let's see what he has to say.
"We need to understand what's gone on here."
Labour leader Sir Keir Starmer said more and stronger lobbying rules are needed because the current system "is not working".
He said ministers were being lobbied for "massive" procurement contracts "involving millions sometimes billions of pounds".
He added: "Increasingly we're seeing a murkier and murkier picture whether it's the way contracts are handed out, the lack of due process or the lobbying which is not a revolving door, it's an open door now in to government".
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