The new Bishop of London, who is a trained nurse, senior civil servant and priest, said she has "one vocation, to follow Christ and make him known", a belief she has held since the age of 16.
The Bishop of Crediton was born Sarah Elizabeth Bowser to father Michael and hairdresser mother Ann, who almost influenced a teenage Mullally to follow in her footsteps, until the prospect of becoming a nurse enticed her.
Raised and educated in Surrey and London, she is no stranger to making history.
She took on several leadership positions, culminating in her appointment as chief nursing officer for England in 1999, a move which she once said "caused a bit of a stir".
At the time, she was the youngest ever to take on the role.
As the Government's chief nurse, she advised senior civil servants and then prime minister Tony Blair on NHS policy.
Her proudest achievement, she said, was introducing "the first patient survey in Europe" and igniting a culture change in the NHS "in trying to understand patients' experience".
She was ordained in 2001 but took on ministry full time in 2004.
The following year she was made a Dame Commander of the British Empire for her outstanding contribution to nursing and midwifery.
In 2015, the senior civil servant was appointed Bishop of Crediton in the Diocese of Exeter, before rising to the top ranks of the Church.
On her leadership style, the Anglican bishop said she was "frisky, agile, vulnerable and irritating" like a flea - a concept introduced by philosopher Charles Handy in The Elephant and the Flea.
But the bishop, who "reads a lot of theology" also counts the works of Archbishop Desmond Tutu and writer William P. Young as sources of inspiration, as well as Mary Seacole, a mixed-race businesswoman and nurse who treated British soldiers in the Crimean War.
A "proud" mother to Grace and Liam, both in their twenties, Bishop Sarah said her two children give her stability and, with her husband, have been "very supportive".
She once said in an interview with the Church Times that she "couldn't do what I do without them".
Her appointment is likely to send shockwaves through the diocese of London, which has become divided over the issue of women priests and bishops.
The previous Bishop of London, the Rt Rev Lord Richard Chartres, chose not to ordain women in an attempt to appease traditionalists.
But the ongoing tensions over tradition are unlikely to faze Bishop Sarah, who put diversity and inclusion - of gender, race and sexuality - at the heart of her first address as the 133rd Bishop of London.
She said that she was "very respectful of those who cannot accept my role as a priest or bishop", but also suggested that for the Church to say relevant, it had to represent the communities of which it is at the heart.
"That means increasing churches that are led by priests that are women, who come from black, Asian and minority ethnic groups," she said as she delivered her speech at St Paul's Cathedral.
As a member of the Church of England's National Safeguarding Steering Group, Bishop Sarah said she would "continue to see that we have a culture which is safe, where there is no place for abuse", in the church.
