Thomas Peter Thorvald Kristian Ferdinand Mortensen was born in Skaarup, Skanderborg, Denmark, on 16 August 1882, to Jens Carl Martinus Mortensen, a tailor, and Maren Therkildsen Thyboe. The youngest of six siblings, he was baptized in Fruering Church on 26 December 1882. Besides his baptismal record, other records used for validation of his age include the 1890 and 1901 census enumerations in Denmark, and church confirmation in 1896.

Following in his father’s footsteps, Mortensen became a tailor. In 1903, in hopes of finding more job opportunities, he migrated to the United States and lived in various areas, and worked in various trades including as a milkman and in a canning factory. His job as a milkman required him to wake up at 1:30 AM, and drive a horse-drawn carriage from house to house to deliver the milk. In Denmark, Mortensen had worked as a farmhand and apprenticed as a tailor. He was married, though only briefly, and never had children. As an avid sailor, he enjoyed making sailing trips from Texas to Florida. However, in 1962, when he was 79 years old and living in Texas, Hurricane Carla destroyed his boat, leading to him making the decision to once again return to Denmark for the first time in nearly 60 years. Upon returning, he reconnected with some family members he had lost contact with, and was interviewed in a local Skanderborg newspaper, where the 79-year old Mortensen was quoted as saying, “where I hang my hat is my home. I have my carriage and rent a few rooms and a kitchen where I arrive. I manage the cooking myself, and since no one loves me, I have neither family nor home.”

In 1968, at the age of 86, Mortensen once again returned to Denmark to celebrate the Rebild Festival, which would mark the final time he visited his homeland. In 1978, at the age of 96, he drove his old Ford car to San Francisco, arriving at a Danish-American nursing home in Aldersly.

Mortensen enjoyed an occasional cigar, and, very unusually for a supercentenarian, was known to show a bit of feistiness even in his final years. In August 1997, for instance, he expected to get the Guinness “oldest person” title, but on 14 August 1997, Guinness named Marie-Louise Meilleur of Canada, age 116, the “world’s oldest person”. Mortensen, angered, said they “just did that to spoil my birthday” (he turned 115 only two days after). Later, however, when he found out that fellow American Sarah Knauss was older too (116 as well), he said “C’est la vie” (translated: “That is life”). Mortensen could have been given the title of “oldest living man”, however, but Guinness did not officially implement that category until mid-2000.

Mortensen died in San Rafael, California, USA, on 25 April 1998.