John Armstrong joined the 2nd Battalion 84th Foot 2nd April 1809 from the Derby Militia for limited service of 7 years and a Bounty of £5/5/-. He was immeadiately given the rank of Serjeant, which he would retain throughout his service

15th July 1809 he sailed with the Battalion in Captain Burn’s Company from Harwich on the Expedition to the Scheldt, where he would succumb to the ‘Walcheren Fever’. Embarking for England 16th September 1809, he was admitted to Ipswich General Hospital until rejoining the Battalion 1st January 1810.

25th July 1810 he was once again sick in England; whilst the Battalion was serving in Ireland. On regaining his health he was recruiting at Ripon, where the Regimental Depot was situated.

28th March 1811 he again joined the Battalion in Ireland for a brief spell, before returning to Depot 7th May 1811, subsequently moving with the Depot to Hull, where he remained recruiting throughout the next three years. He would not serve in the Peninsula Campaign.

21st January 1813, he received £6/8/9 Prize Money for the Walcheren Campaign

In late 1813 the rank of Colour Serjeant was created.

When the 2nd Battalion returned to Ireland following the end of hostilities, Serjeant John Armstrong once more joined them for the Muster 25th October 1814. He would then be promoted Colour Serjeant 28th November 1814

21st August 1815 Lieutenanat John Cooper was Court Martialled and Cashiered, part of the 1st charge being that he failed to repay a debt of £20/12/6d to Serjeant Armstrong. It is quite possible that they were both part of the same recruiting party – See entry John Cooper in Officers

16th February 1816, he was shown on Muster as discharged, he however re-enlisted 1st April 1816 in his original rank of Colour Serjeant for unlimited service with a bounty £4/-/2d

19th December 1818, he was discharged on reduction of the Battalion