Aged 19, Archibald Duffield Eden had his first commission in the 26th Regiment, as an Ensign on the 21/02/1860. Within two years he became Lieutenant (08/07/1862) Interestingly, his father William Hassell Eden became Colonel of the 90th LI later in the same year. This does not seem to be anything more than just coincidence, although it does disprove the theory of nepotism, as his career progressed at an average pace, despite his connections.

The army provided many opportunities to travel and this may explain why he did not marry for a further ten years until 1870, when his first son (Archibald James Ferguson Eden) was born.

The 26th were then sent to serve in India, Bombay, Calcutta and Abyssinia (now known as Ethiopia) for his service there, he received the Abyssinia medal of 1868.

His second son was born in 1873 (William Rushbrooke Eden) three years before his next promotion to the rank of Captain. The 26th and 90th amalgamated in 1881 and at that point he was a major in the new 1st battalion.