Emma Walmsley, who has described herself as "extremely competitive", will become the most powerful woman in British business, as she steps in as boss of the UK's fifth largest company.
She has run GSK's Consumer Healthcare unit since 2010.
But she lacks any previous experience in a chief executive role and has no background in pharmaceuticals.
Consequently she will start on a pay package 25% lower than her predecessor Andrew Witty, who retires on Friday.
GSK's most recent annual report indicates her basic salary will be about £1m, in contrast with Mr Witty's £1.1m. Her pension and potential for earning through bonuses and long-term performance targets are also lower.
"Taking into account the fact that this is Emma's first chief executive role, reductions have been made to all elements of her remuneration package in comparison to Sir Andrew's," the report said. "Constructive feedback" about remuneration from shareholders had also played a role, it said.
GSK is the fifth largest company traded on the London Stock Exchange and as head of its Consumer Healthcare business Ms Walmsley was responsible for brands such as Sensodyne toothpaste and the malted drink, Horlicks. Before joining GSK she spent 17 years at French cosmetics giant L'Oreal.
'Bias for action'
Ms Walmsley originally studied classics and modern languages at Oxford before moving into business, taking up a management and marketing role at L'Oreal, which included several years at its Shanghai office.
Nicholas Hyett, equity analyst at Hargreaves Lansdown, said he did not see her lack of scientific background as a disadvantage.
"It's not as if GSK lacks people with a pharma background," he said.
"If anything, there's more strength in her having a firmer grip on the consumer business, as that's something that... maybe wouldn't be done as well by someone from the pharma side."
Ms Walmsley has described herself as having a "bias for action" and as highly competitive. She also said she had been inspired by other business leaders such as Alibaba founder, Jack Ma, and Facebook's chief operating officer, Sheryl Sandbrook.
She is married with four children, but has said in the past that she did not see herself primarily as a woman in business but as a business person.
She said that she was supported through her maternity absences and was aware of her responsibility to keep supporting young talent as well as acting as a role model to encourage young women "to stay ambitious, aim high and think big".