Loudon, was born in Leeston, New Zealand[1] and relocated to Sydney during his youth. He attended Sydney Grammar School and gained entry to Sydney University to study medicine.[2] The Sydney club rugby competition started up again after World War I in 1919 and Loudon was captain of the Sydney University Football Club the premiership winners, also winning himself a University Blue that year. He captained the side in matches against an AIF Rugby XV who toured NSW and Queensland and led by former Wallaby Lieutenant Willie Watson did much to rekindle public attention in rugby union in the Australian eastern states.[2]
Loudon made his representative debut on the New South Wales tour to New Zealand in 1921, playing in nine of the ten tour matches scoring two tries, nine conversions and four penalty goals for 36 points, the second top-scorer of the squad. With no Queensland Rugby Union administration or competition in place from 1919 to 1929, the New South Wales Waratahs were the top Australian representative rugby union side of the period and a number of their 1920s fixtures played against full international opposition were decreed by the Australian Rugby Union in 1986 as official Test matches. Thus Loudon's appearance against a New Zealand XV at Lancaster Park in September 1921 marked his international Test rugby debut.[1] So decreed also were the three matches Loudon played for New South Wales in 1922 when a New Zealand Māori rugby union team toured to Sydney.[1] He captained the side in the second match of that series, thus qualifying as an Australian national rugby union team captain with four international rugby caps appearances to his credit.[2]