Thomas Schreiber was convicted yesterday following a three-week trial at Winchester Crown Court of stabbing the pair at the baronet’s Dorset country estate on April 7 this year.
A cry of “yes” could be heard from the public gallery, where some members of the family were sitting, as the verdict was announced.
Others, including the defendant’s mother Anne Schreiber who he was convicted of trying to kill, watched by video-link.
The jury of ten men and two women reached a majority verdict of 11 to one on both counts after four hours and 51 minutes of deliberations.
Adjourning the case for sentencing on Monday, the judge, Mr Justice Garnham, told the defendant: “The only sentence I can pass is of life imprisonment, but for the offence of murder I have to set the minimum number of years and I also have to sentence you for the attempted murder of your mother.”
Speaking to the jury, he added: “I am extremely grateful to you all for the evident care you have taken in this case, it’s been a difficult case to listen to.”
Sir Richard’s family said in a statement: “How could any family recover from such a sudden and devastating loss.
“We can never bring back Sir Richard, but his spirit will very much live on, alongside the very happy memories we have of our incredible father, brother and grandfather.
“His values of being warm, generous and compassionate to everyone he met will be carried forward by future generations, and will never be extinguished.”
The 35-year-old defendant admitted carrying out the “horror show” knife attack on the pair on the eighth anniversary of the death of his alcoholic father, David Schreiber, after he said his mother shouted at him for being “drunk like his father”.
He told the court: “I just went completely crazy. I completely snapped and I just launched for her and began attacking her uncontrollably.”
He had admitted manslaughter, but denied the murder of the baronet, saying he had “lost control” of himself and had not intended to hurt him or his mother.
Ms Schreiber suffered up to 15 separate injuries on her head and body, including a partially severed spinal cord that has left her paralysed.
Sir Richard suffered three deep wounds to his face and five to his chest, up to 12cm deep.
Schreiber had also pleaded guilty to driving a Range Rover dangerously on the A303, A4 and M3 when he failed to stop for armed police in a 135mph pursuit into central London.
As he was detained in Chiswick, the defendant stabbed himself in the chest and told officers: “Please kill me now, please just shoot me.”
The attack happened at Sir Richard’s Moorhill estate near Gillingham, Dorset, which he shared with the Schreiber family following the separation of the defendant’s parents.
The trial has heard the defendant was angry with his mother for “abandoning” his father, who suffered from depression, to move in with Sir Richard and accused her of being a “gold-digging bitch”.
Schreiber, an aspiring painter, also felt Sir Richard treated him unfairly compared to his two sisters despite him receiving a £1,000 monthly allowance and £100,000 towards buying a house.