Sæther departed from his native municipality for Oslo in 1868. By December 1870, he had enlisted in the Norwegian Hunter Corps, and served from 1 October 1871 to 1 May 1874. At some point, he was transferred to His Majesty's The King's Guard.

In 1876, Sæther was promoted to corporal, and two years later he became a sergeant and in the same year, a lieutenant.

Additionally, he managed the finances of His Majesty The King's Guard for many years.

Sæther married Agnes Lovisa Charlotta Widin (1854-1892) on 7 October 1882 in Stockholm. They had 4 children:

Elsa Maria (1884-1892)

Martha Lovisa (1887)

Arne Nicolaus (1889-1889)

Astra Maria (1892)

His daughter, Astra Maria, became an associate professor and a lecturer at the high school in Horten.

When his battalion returned to Norway in 1888, he was offered employment as a cadet in one of Stockholm's guard regiments or in the city's civil administration because of his excellent reputation.

He declined the offer and settled Oslo, where he would remain for most of his life. After 39 years of military service, he resigned in 1909.

Sæther recieved several awards for his service:

The Prussian Medal of Merit in 1890, Oscar II's Medal of Merit in 1900, the Order of the Red Eagle in 1906, the Legion of Honor in 1908, and His Majesty The King's Guard Medal (in silver) on his 90th birthday in 1938.

At the time of his 104th birthday, he was visiting his daughter, Astra, in Horten; on his 107th birthday, he was still in good health.

Mathias Nicolaus Sæther passed away on 7 January 1957 in Horten, Vestfold, aged 108 years and 78 days. At the time of his death, he was the world's oldest living person

(Age 79, just before his 80th birthday)

(On his 100th birthday)

(Around the time of his 104th birthday)