David Redfern, 46, a bank IT worker, of Barmouth, Gwynedd, had refused to call an ambulance as frail Margaret Barnes lay dying outside his five-storey townhouse last July.
He had denied murder or manslaughter but was convicted of murder by a jury at Caernarfon crown court.
A Caernarfon crown court jury took 14 hours and 30 minutes to convict him of murder on Wednesday.
Adjourning sentencing until Friday, Mr Justice Bourne said his task was to decide the minimum term before parole may be considered.
Prosecuting, Michael Jones KC said Redfern was a 21-stone, 6ft, angry bully.
He had dragged Mrs Barnes, a retired factory worker from Birmingham, downstairs by her ankles after finding her in his bed.
Home Office pathologist Dr Brian Rodgers said the attack caused “deep tearing” of her liver, broken ribs and internal bleeding.
Redfern, who had drunk six or seven pints and a gin and tonic, claimed Mrs Barnes became “agitated” when he made a 999 call to police.
He said he went into his bedroom with his fiancee to find Mrs Barnes in the bed drinking and smoking.
When he told her to get out she said it was her room.
“An intruder in our bedroom, in our bed, I just wanted the police to turn up and remove her,” he said.
"Mrs Barnes became agitated that the police had been called and I believe she further demanded we get out of ‘her’ room," he told the jury.
He said he started guiding the pensioner towards the door but was pulled off balance and they both fell to the floor.
“I fell directly on top of Mrs Barnes."
But a camera recording captured him saying: "I’m sorry for kicking her in the ribs and dragging her down the stairs.”
A witness described him ranting like a “raving lunatic”.
After the verdict Mrs Barnes’s family said: “Margaret, who was a much-loved wife, mother and grandmother, has now been gone for eight months. As a family it has been the hardest time of our lives. It has been especially difficult for Margaret’s husband, who had been her partner for 56 years. We now have some sort of closure on what has happened, however, no length of sentence will ever fill the void that Margaret has left behind.
“As a family we would like to thank the police for their hard work in putting the case together.”