The promotion was a birthday gift for the long-standing MP, who celebrated his 61st birthday on Wednesday.
He was elected to the role following the shock resignation of Premier Ted Baillieu.
Born and raised in Geelong, Dr Napthine started his schooling in Winchelsea before going on to study veterinary science at the University of Melbourne.
He worked as a veterinarian in regional Victoria with the Department of Agriculture and Rural Affairs while becoming involved with the Liberal Party.
In 1984, he became a state council representative and the following year became Hamilton branch president until 1988, when he was elected as the member for Portland.
In 1992, when Jeff Kennett was elected to the premiership, Dr Napthine was appointed parliamentary secretary to the health minister.
From there, he rose through the ranks, being promoted to youth and community services minister and then treasurer.
His stint as treasurer was short lived after the Kennett government unexpectedly lost the 1999 election.
The resignation of Mr Kennett paved the way for Dr Napthine's appointment to the Liberal leader role.
He went on to serve as shadow minister in various portfolios until the Liberals won the 2010 election under the leadership of Mr Baillieu.
Stepping into his new role, Dr Napthine praised his predecessor.
He said was proud to call him a friend and proud to have been a part of his team.
"I will lead the party to the next election," he said.
Maybe this time, he will.