Thaxter and his mother, June Buttle, were said to have chopped up Mr Groome’s body in January 2010, and burnt it on a bonfire.
Buttle is serving a 16-year sentence after admitting manslaughter, last year.
After the case at Sheffield Crown Court, crown prosecutor Mark Langan said: “Six years after his death, justice has been done and today’s verdict means that both of Tommy Groome’s killers are now facing considerable sentences for their crimes.
“Thaxter and his mother, June Buttle, killed Tommy Groome some time in early January 2010.
“His body has sadly never been found and Thaxter and Buttle sought to get away with their crime by weaving a tangled web of lies and deception to cover up his death.”
At the trial, two witnesses said Buttle, of Doncaster, had confessed to killing Mr Groome.
Mr Langan said Buttle had “told one witness in some detail how she and her son had dismembered his body and disposed of it on a bonfire”.
He added: “Their motive was a combination of greed after Mr Groome had been awarded a substantial sum of compensation, and revenge for the violence which had been a prominent feature of their relationship.”
At the start of the trial, Dafydd Enoch QC, prosecuting, said Mr Groome had planned to emigrate to Portugal to open a bar with his windfall. Buttle and her son then came up with a plan to kill Mr Groome and pretend he had gone abroad alone.
The court heard Mr Groome and Buttle had a “stormy and at times violent” relationship.