Chris Langham had pleaded not guilty to all the charges
Langham, of Golford, Kent, said he was only studying the porn as research for a television drama - but he was convicted at Maidstone Crown Court.
However, the 58-year-old actor has been acquitted of charges of indecently assaulting an underage girl and two counts of a serious sexual offence.
Judge Philip Statman remanded the Bafta winning comedy performer in custody for sentencing on 14 September.
The jury of seven men and four women deliberated for about two hours and 40 minutes before they returned their verdicts.
'No knowledge'
Langham claimed in court that he had looked at the images while formulating a character for the BBC series Help.
He said he pleaded not guilty because he did not want to be called a paedophile.
I have been found guilty on charges I have made admission to from the moment of my first arrest
Chris Langham
Highs and lows for Langham
During his time in the witness box, Langham said when he was eight he had been the victim of abuse while on holiday in Canada. He said he did not want to go into any detail, but he felt "deep, deep shame" about what had happened.
Langham, who was convicted on 15 charges of downloading child porn, also told the jury victims of abuse were his "brothers and sisters" and he felt no shame in looking at child pornography.
When asked by the prosecution why he had saved images on to his computer, he replied: "I thought if I could become angry enough I might be able to break this problem I have in accessing this stuff myself."
The actor was acquitted of indecently assaulting an underage girl, who is now 25 and cannot be identified for legal reasons.
She had claimed in court that he had taken her virginity at the age of 14 and they had also had sex at his home and in his dressing room.
Langham denied all the charges. But he did say he had had one sexual encounter between them when she was 18.
The jury were directed to clear the actor of four other indecent assault charges part way through the trial.
'Harrowing and disturbing'
In a statement read out by his solicitor, Langham said: "I am absolutely delighted my name has been cleared of all the charges I have consistently denied.
"I have been found guilty on charges I have made admission to from the moment of my first arrest.
Viewing such images simply perpetuates child abuse
Det Insp Derek Cuff
Langham's public fall from grace
"I am afraid I am unable to comment further until sentence on the 14 September."
Det Insp Derek Cuff, of Kent Police, said: "Let us not forget that child abuse images circulating on the internet are real situations involving real children, who are sexually abused for other's gain and self-gratification.
"Viewing such images simply perpetuates child abuse."
Ken Goss of the Crown Prosecution Service, said: "The images found on his computer were not child pornography. They were horrific images, still and video clips, of children being sexually abused. Chris Langham actively searched for those images."
The British actor Chris Langham has been found guilty on 15 counts of downloading child pornography. A jury of four women and seven men took two hours and forty minutes to reach a verdict at Maidstone Crown Court. Judge Philip Statman said that Langham will remain in custody till his sentencing on September 14.

Langham, 58, from Goldford, Kent claimed that he had been studying porn sites, researching for a character called, "Pedro," for the television series ''Help''. However, co-star Paul Whitehouse claimed he had no knowledge of this research. Langham refused to answer questions about certain images found on his computer.

During his time in court, Langham claimed that he was abused at the age of eight during a holiday to Canada. He also said his brothers and sisters were abused, and that he felt no shame at looking at child pornography. Langham said he pleaded not guilty because he did not want to be labelled a, "Paedophile".

Langham however was cleared of charges of indecently assaulting an underage girl. The girl claimed that Langham took her virginity at the age of 14, but Judge Statman said, "The Crown accepts that there is no evidence to support these particular counts on the indictment."

Langham said, in a statement read out by his solicitor, "I am absolutely delighted my name has been cleared of all the charges I have consistently denied. I have been found guilty on charges I have made admission to from the moment of my first arrest. I am grateful to the jury for their careful deliberation and I would also like to thank the press for their restraint in not making the lives of my wife, children and family difficult during this time and I implore them to continue to grant my family the privacy they need. I am afraid I am unable to comment further until sentence on the 14 September."

Derek Cuff, Deputy Inspector of Kent Police said, "Let us not forget that child abuse images circulating on the internet are real situations involving real children, who are sexually abused for other's gain and self-gratification. Viewing such images simply perpetuates child abuse. The jury had to view some images that had to be selected carefully - but what they saw and heard in evidence is harrowing and disturbing. I think this itself puts some measure of perspective on the level of child abuse we are dealing with."

This will affect the filming of the next series of the satirical sitcom ''The Thick of It'', where Langham plays Hugh Abbot MP. The last two special episodes did not star Langham due to his arrest. Writer Armando Iannucci hoped that if Langham was found not guilty, he would be able to appear in his show again.