Michael Stone has been found guilty of the murders of Lin Russell and her six-year-old daughter Megan, as well as the attempted murder of Josie Russell.
The jury took more than 10 hours to find Stone guilty by a majority of 10-2, he was given three life sentences.
Stone was originally found guilty of the attack in 1998, but a retrial was ordered earlier this year.
Lin and Megan Russell were tied up and beaten to death with a hammer in July 1996, while walking on an isolated footpath in Kent, Josie Russell managed to survive the assault.
The retrial was ordered after one of the key witnesses in the first trial - a prisoner who claimed that Stone had confessed to him - admitted that he had lied to the jury.
Stone, 41, had previous convictions for violent crime, including one incident when he attacked a man with a hammer.