af Uhr was born on 25 January 1892 in Bondkyrka, Uppsala County, Sweden, the son of builder Otto af Uhr and Anna (née Ferm). He passed "studentexamen" in Uppsala on 31 May 1911 and became a volunteer in the Life Regiment Grenadiers (I 3) on 2 June 1911. af Uhr was transferred to the Jämtland Ranger Regiment (I 23) on 1 June 1912. About a month later, af Uhr finished 23rd in the high jump competition during the 1912 Summer Olympics.