The former computer science student committed most of his crimes when he was still a teenager, but the complexity of the case meant it took more than four years to bring Qaiser to justice.
Using the online name K!NG, he posed as a legitimate businessman to buy large amounts of advertising space on pornographic websites.
But when users clicked on the adverts their computers were exposed to a highly sophisticated virus known as Angler software.
The infected computers then displayed a message which purported to be from the local law enforcement agency and warned the user that they had committed an offence and had to pay US$1000 (£767).
The victims spanned at least 20 countries and the crime group for which Qaiser was working are thought to have generated millions of pounds in ransoms.
The money was paid through an online currency which could then be put onto pre-loaded credit cards.
It was then withdrawn in cash abroad by others in the network and redirected through online currency services.
While some of the advertising companies Qaiser was buying space from turned a blind eye to his activities, ones who challenged him were targeted with cyber attacks including threats that they would be bombarded with indecent images of children.
The resulting lost business ended up costing some companies hundreds of thousands of pounds.
Qaiser had initially acted alone but later contacted the Russian crime syndicate offering his services as an English speaker and expert in the online advertising industry.
Sentencing him to six and a half years, Judge Timothy Lamb QC, said: "The harm caused by your offending was extensive - so extensive that there does not appear to be a reported case involving anything comparable.
"All the constituent offences were part and parcel of your role as the self-styled 'K!NG' of the internet.
"It has been asserted on your behalf you are remorseful. I have seen no outward expression of that."
Qaiser, who initially denied the crimes and claimed he had been hacked himself, remained expressionless as he was sentenced.
Nigel Leary, a senior NCA investigating officer, said: "Zain Qaiser was an integral part of this organised crime group generating millions of pounds in ransom payments by blackmailing countless victims and threatening them with bogus police investigations.
"The FBI and the US Secret Service have both arrested people in relation to this global malware campaign.
"The investigation demonstrates that cyber criminals cannot operate from behind a veil of anonymity and that the NCA has the tenacity and specialist skills to catch them and bring them to justice."
Qaiser entered guilty pleas to an 11-count indictment, admitting three counts of blackmail; three counts of fraud by false representation; four counts of doing an unauthorised act with intent to impair the operation of a computer; and one count of possessing criminal property.
