Paul Taylor, of Valentine Close, Fareham, Hampshire, was also found guilty of three rapes, an attempted rape and a serious sexual offence.
Today, at Chelmsford Crown Court, Judge Mr Justice Owen said: "I think it is clear there was a sexual element to the murder of Sally McGrath.
"The evidence revealed a pattern of behaviour in which you lured your victims in under the pretence of giving them a lift home.
"You then drove them to a rural or isolated area and forced yourself upon them.
"Your predatory sexual attacks took place over 30 years ago but your victims have been left scarred for life.
"I have no doubt the brutal murder of Sally McGrath followed the same pattern and resulted from her resistance to your sexual demands."
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Mostly dry with bright or sunny spells
Thames Valley Police are appealing for help in tracing two teenage girls missing from Milton Keynes since Saturday afternoon.
A fine and dry day in the Anglia region on Monday with some good sunny spells.