Steve Whittamore dismissed the newspapers' claims that only a small amount of what he did was illegal.
The Mirror Group Newspapers' denies senior executives knew about the practices and failed to stop them.
Another key witness also described the MGN as an "organised crime group".
Mr Whittamore was giving evidence to the civil trial into breaches of privacy of the Duke of Sussex and other figures.
It is alleged journalists from the newspapers obtained private and confidential information about people's lives through a variety of unlawful means between 1991 and 2011 - including accessing voicemail messages on their phones.
Mr Whittamore said he was in no doubt his newspaper customers were aware the information was obtained through 'blagging'.
If the information had been available legitimately, he said, the newspapers would not have needed his help.
At one point, he claimed he used an email address firstname.lastname@example.org to send invoices to his journalist customers, according to his witness statement.
MGN told the court this address had only been used once.
The company has admitted unlawful methods were used to gather stories, but denies that executives and editors knew.
Mr Whittamore was convicted of breaching information laws in 2005 after an investigation during which all his records and payment invoices were seized.
Cross-examining him, Andrew Green KC, for the publisher, suggested that some of these records suggested he had also provided legitimate research for journalists.
"Would you accept the majority of the work you did for MGN was lawful?"
"No," Mr Whittamore responded.
"Perhaps you weren't quite as bad as you now wish the court to believe," Mr Green said, referring to Mr Whittamore's invoices.
"Well it wasn't me doing the checks," Mr Whittamore responded. His business involved dealing with a network of 'subcontractors' who were experts in blagging confidential information.
He was asked about one invoice for a "mobile TP conversion" under the name of Tom Newton-Dunn, then a young journalist for the Daily Mirror, but later political editor of the Sun, and now a presenter on Times Radio.
He told the court this meant getting a name and address for a mobile number, information which usually can't be obtained legitimately by journalists.
Steve Whittamore's statement claims he offered to obtain criminal records, benefits records, or bank details.
His services included providing telephone bills, ex-directory numbers and vehicle registration details.
A key witness in the civil trial also described the publisher of the Daily Mirror as an "organised crime group" similar to "drug gangs and fraud factories".
Graham Johnson, a journalist convicted of phone hacking, is now helping the claimants bring their case against MGN.
He said the company had been involved in "systematic crime" and a "cover-up which has gone on for over a decade".
MGN has admitted unlawful methods were used to gather stories, but denies that executives and editors knew.
Mr Johnson said as a journalist for tabloid newspapers he had investigated "drug gangs in Liverpool, fraud factories in south-east London, street gangs in Birmingham."
He said MGN was "no different from an organised crime group. This is a true crime story which is constantly evolving."
He was being questioned by Mr Green about his role in obtaining evidence of unlawful information gathering and passing it to the claimants.
Mr Johnson said at first he tried to get media organisations including the BBC, ITV and newspapers to report on the methods used by tabloid journalists but they "didn't want to write the story."
So, he said "if the claimants asked me for information I also started giving them information. I had no other way of getting that information out there, because no-one would take the story."
He said in effect he was "letting the court be my newspaper."
An American private investigator Daniel 'Danno' Hanks also gave a statement that he was "sure that the UK newspapers and their journalists knew what I was doing, not least because they could see the product of what they commissioned".
Mr Hanks told the court he had been convicted of at least 20 criminal offences but had decided to provide evidence of how newspapers used his services to "right the wrongs before I pass."
He specialised in using US databases to get celebrity information including social security information, voting registration records, phone records and details about vehicles.
Giving an example, he said in his witness statement that he had obtained phone records for a boyfriend of the singer Kylie Minogue.
He told the High Court he had a higher level of access to databases as a private investigator, but used this to provide information for journalists, which was illegal.
"I am able to get round the restrictions by simply accessing the database under the false pretence of legitimate PI work," he said in his witness statement.
After the phone hacking scandal broke in the UK, he said in his statement: "Many of the tabloid newspapers appeared to be panicking about using anyone who was a private investigator."
But, he said: "They only wanted to distance themselves on paper."
He was "encouraged" to send invoices using the company name "British American News Service", rather than his actual names "Detective Danno" and "Backstreet Investigations".
"I was never asked to change my product or methods which they knew to be illegal," he said.
Prince Harry is expected to give evidence at the trial in June. He is among four people whose claims are being heard in the trial as "representative" cases of the types of allegations facing the publisher. They will also help the court set the level of damages MGN should pay if the claimants win.
Others involved are Coronation Street actors Nikki Sanderson and Michael Turner, known by his stage name Michael Le Vell, and comedian Paul Whitehouse's ex-wife Fiona Wightman.