In 1967, Felix Dennis's career started as a street seller, selling copies of the underground counterculture magazine Oz magazine on the streets of London’s Kings Road. The magazine printed a mixture of left-field stories, discussions of drugs, sex and contentious political stories. He then gained an informal apprenticeship in magazine design with Jon Goodchild, Oz's first art director, who later moved to Rolling Stone in California.

After passing out at a friend’s party, he woke up in a flat shared by several young women in Walsingham Mansions on the Fulham Road and moved in, paying rent by cleaning the flat. What would become a massive collection of '60s and '70s graphic art began in a small way with the posters of Martin Sharp, one of the originators of Oz magazine.

In 1969, Felix wrote a world exclusive for Oz, the first ever review of Led Zeppelin‘s debut album. He was quickly promoted to co-editor and became involved in the longest conspiracy trial in English history over the infamous "Schoolkids Oz" issue. While Richard Neville was on holiday, Jim Anderson and Dennis had invited 5th and 6th-form kids to edit the issue. They included a sexually explicit Rupert the Bear cartoon strip, which proved too much for the authorities and resulted in the arrest of Anderson, Neville and Dennis, who were charged with "conspiracy to corrupt public morals". The Oz offices in Princedale Rd, Notting Hill and the homes of its editors were repeatedly raided by Scotland Yard's Obscene Publications Squad.

Dennis recorded a single with John Lennon to raise money for a legal defence fund.

At the conclusion of the trial, the 'Oz Three', defended by John Mortimer, were found not guilty on the charge of "Conspiracy to deprave and corrupt the Morals of the Young of the Realm", but were convicted on two lesser offences and sentenced to imprisonment. Dennis received a more lenient sentence than his co-defendants because he was, in the opinion of the judge, "much less intelligent" and therefore less culpable. These convictions were later quashed on appeal. Dennis later told author Jonathan Green that on the night before the appeal was heard, the Oz editors were taken to a secret meeting with the Chief Justice, Lord Widgery, who told them that they would be acquitted if they agreed to give up work on Oz. It is alleged that MPs Tony Benn and Michael Foot had interceded on their behalf.

In 1973, following acquittal by the Court of Appeal, Dennis went on to found his own magazine publishing company. With the rising popularity of martial arts with the film Enter the Dragon, Dennis's Kung-Fu Monthly became a success just two years after the Oz trial, making over £60,000 in its first year.

Dennis was the second publisher of Personal Computer World which he later sold to VNU, and established MacUser which he sold to Ziff Davis Publishing in the mid-eighties. In 1987, he co-founded MicroWarehouse, with Peter Godfrey and Bob Bartner, a company that pioneered direct IT marketing via high quality catalogues. The computer mail order company eventually went public on the NASDAQ in 1992. At the time it had 3500 employees in 13 countries with worldwide sales in 2000 of $2.5 billion. It was sold to a private investment group in January 2000. This created the bulk of Dennis' personal wealth. Dennis launched further successful IT titles Computer Shopper and PC World.

In 1995, Dennis Publishing created Maxim, a title that began on the back of a beer mat and became the world's biggest selling men's lifestyle magazine and global brand. In 1996, Dennis acquired a majority stake in what is now Dennis Publishing's flagship brand The Week which is published in the UK and US and translates to a global circulation of over 700,000 (ABC audited). Over the following years it purchased the remainder of shares from original founder Joylon Connell and Jeremy O’Grady. 2003 saw the purchase of IFG Limited (I Feel Good) from Loaded founder James Brown. The purchase involved titles Viz, Fortean Times and Bizarre being added to the Dennis Publishing stable. In June 2007, Dennis sold his US magazine operation which published the magazines Blender, Maxim and Stuff to Alpha Media Group for a reported $250 million USD although exact details were never disclosed.

In 2008, Dennis Publishing established digital magazines iGizmo, iMotor and Monkey along with the purchase of The First Post from the Kensington based First Post Group for an undisclosed sum. The award winning online magazine which gained a D&AD nomination for viral marketing was headed-up by former Telegraph editor Mark Law and Evening Standard editor Nigel Horne. This title later morphed into The Week.

In 2013, Dennis remained the sole owner of Dennis Publishing, with offices in both London and New York City. It currently holds over 50 magazine titles, digital magazines, websites and mobile sites in the UK including The Week, EvoAuto ExpressCarBuyerPC Pro, Computer Shopper and Viz. Its flagship brand The Week continues to be published in the US alongside Mental Floss magazine.