Miss Sarah Agnes Stap was born at sea off the coast in the Bay of Bengal in the Indian Ocean on 1 August 1864.

She was the daughter of Henry Stap (b. circa 1829), a native of Skipsea near Bridlington in east Yorkshire, and his wife Stella, née Cawkwell (b. 14 October 1835), a native of Bermondsey, London. The couple were married in Mile End, London on 31 August 1861 and Henry was then described as a master mariner. Sarah was born on board one of the vessels he commanded and he reportedly held several commands in the White Star and Leyland Lines.

Sarah was one of five children, her siblings being: Stella (1862-1943, later Mrs Edward Wood Tagg), Edith Mary (1868-1953), Henry (1871-1933) and William Cawkwell (1875-1923). Her sister Stella was also born at sea whilst her sister Edith was born in London. Both of her brothers were born in Essex, her brother Henry later working as teacher where he was a master at Oake's Institute in Liverpool for twenty-five years.

Sarah first appears on the 1871 census when she and her family are living at Willesly Road, Wanstead, Essex. Aged 16 and without profession by the time of the 1881 census, Sarah and her family were by now living at 3 St Philips' Terrace, Kensington, London. The whole family later shifted northward and settled in Seacombe, Wallasey, Cheshire, being listed there at 48 Church Street on the 1891 and 1901 censuses. Sarah would be absent from the latter, perhaps at sea. Her mother died in 1903 aged 67 and Sarah, her widowed father and several siblings appear on the 1911 census living at 41 Bidston Avenue, Birkenhead.

Sarah Stap had been on the maiden voyage of the Baltic and Adriatic and also served on the Celtic, and most recently the Olympic, then as a nurse.

When she signed-on to the Titanic on 9 April 1912 Miss Stap gave her age as 31 and address as 41 Bidston Avenue, Cloughton, Birkenhead. She signed on as a first-class stewardess although according to her great-nephew Gordon Stap, she was not a stewardess but a ship's matron. In her position, she received monthly wages of £3, 10s.

Miss Stap later recalled that on the night of the sinking she was in bed at the time of impact and was awoken by a "slight bump" but took little heed, claiming she had become accustomed to "a ship's bumping before" and she lay in bed for a further three quarters of an hour before rising. When she reached the boat deck the lifeboats had already been slung out. Sarah claimed that she owed her survival to a young cabin boy beside her who, when she was told to get into a lifeboat by the crew member in charge of that lifeboat, that there was room for her, she told the young cabin boy that as she was into her thirties and had had her life, he should take her spot. The cabin boy's answer was to simply pick her up, and put her in the lifeboat, which she described as the last-but-one (some historians place her in lifeboat 11). She later recalled the crowded conditions in her lifeboat (over 70 persons) and the bitter cold of the night, the ordeal lasting for over six hours: 

"We could see the lights of the ship slowly disappear beneath the waves one by one, until there remained alone the masthead light. Then suddenly the great ship gave a lurch and disappeared gracefully out of sight. All this time the people on board were shrieking in their death agonies, and the passengers in the boats were under the impression that it was the people in the other boats cheering. Only the members of the crew knew what it was, and we dare not say. After the ship had gone down an explosion rent the air, and the shrieks of the dying were positively awful." - 1912 interview, reprinted Birkenhead News, 3 April 1937

Sarah returned to the sea and maintained her residence at 41 Bidston Avenue for a time; she lost her father on 2 March 1914 when he died aged 85, later being buried in Rake Lane Cemetery in Liscard, Cheshire. After this loss Sarah moved to Egremont in Wallasey where she spent a few brief years. Her 1924 crew card describes her as standing at 5' 3" and with brown hair, grey eyes and a fresh complexion.

By August 1917 Miss Stap was serving as a stewardess aboard the Justicia, another ship that eventually met a brutal demise. Towards the close of the decade, she was again working as a stewardess aboard the Olympic, later transferring to the Baltic and alternating between both ships between 1919 and 1921. On 25 August 1920, she was working aboard the Olympic when that ship entered New York; three of her fellow stewardesses at the time were Mabel Bennett, Elizabeth Leather and Violet Jessop. 

She ultimately transferred to the new flagship Majestic sometime around 1922 and where she would spend the rest of her career. During her tenure aboard the Majestic, she was presented to King George V, Queen Mary and the Duke of Connaught, who shook her hand and congratulated her on her bravery during the Titanic disaster. Her last known recorded voyage aboard Majestic was in September 1930 when she was described as standing at 5' 2", weighing 108 lbs and aged 48 (she was in fact 66 years of age).

Sarah Stap moved from Egremont, Wallasey back to Birkenhead where she spent the last two decades of her life. Her last few years were spent living with her two sisters at 414 Park Road North and she died there in her sleep aged 72 on 27 March 1937. She was buried on 1 April 1937 in Rake Lane Cemetery following a service at the same venue.