Daman Sehra and his uncle Derek Brennan were convicted of the murder of 32-year-old Amjad Khan at Wolverhampton Crown Court.
Mr Khan died five days after suffering serious head injuries when he was beaten up by the two men at a flat in Oldbury in the early hours of June 8 last year.
The flat in Yardley Close, Bristnall Fields, belonged to Sehra's mother Lisa, who is also Brennan's sister.
Mr Khan suffered the injuries that killed him within minutes of being let into the property by Ms Sehra at 12.41am.
Khan went upstairs into the lounge where he met Brennan and Sehra, who refused his offer of shaking their hands.
Brennan approached Mr Khan from behind and put his arms around his neck area, before Ms Sehra intervened – leading Damon Sehra to punch Mr Khan, prosecutor said.
When Mr Khan fell to the ground, he was repeatedly kicked and stamped on and as Ms Sehra left to call police at around 12.47am Mr Khan was hit by a toy and a glass Stella Artois bottle.
Sehra and Brennan then fled out of the window and onto the roof, leaving Mr Khan dying on the floor less than 10 minutes after he arrived at the flat.
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Blood was later found along a skirting board, on a wall, on the inside of a cupboard and on a junction box.
The defendants, who stopped at Asda in Oldbury to buy alcohol in the aftermath of the attack, said Mr Khan had been "forcing" crack cocaine and heroin onto Ms Sehra.
Sehra, aged 23, admitted manslaughter on the ground of diminished responsibility but said it was never his intention to kill or harm Mr Khan and denied murder. Brennan, 33, denied both charges.
Sehra said he attacked Mr Khan over drugs and the way he had spoken to his mother, but prosecutors said the attack was actually in retaliation to Mr Khan "jumping" Sehra in Smethwick at an earlier date.
Meanwhile Brennan admitted grabbing and punching Mr Khan but denied kicking him and said he put Mr Khan in the recovery position after asking Sehra to stop.
Jurors convicted both men of murder on Thursday after deliberating for eight hours.
Judge Michael Chambers QC said: "You've been convicted of a very serious matter, it's important that I assess the appropriate minimum time that you will have to service before your cases are considered by the parole board but in the mean time you must remain in custody."
Both defendants had also been charged with attacking a 15-year-old boy at a different address in Oldbury earlier that night.
Sehra, of no fixed address, was found guilty of attempting to cause grievous bodily harm and had previously admitted a charge of assault by actual bodily harm.
Brennan, of Halesowen Road, Netherton, was found not guilty of attempting to cause grievous bodily harm and not guilty of assault by actual bodily harm.
DCI Gill Davenport, who oversaw the investigation, said: “This was a brutal, ferocious attack which came completely out of the blue, against a man who these two killers had only just met.
“The motive behind it is not entirely clear, but what is clear is that Mr Khan was unable to defend himself against this attack by two men who have a long history of violence.
“Our thoughts are with his family today who have been steadfast through a harrowing trial. I am pleased that they have finally received justice for their son."
The pair will be sentenced at Wolverhampton Crown Court on Wednesday.