Armstrong entered into club management in 2016, joining Blyth Spartans, achieving promotion to the National League North in his first season in charge.
[6] He signed for Newcastle Blue Star in the UniBond Division One North before quitting days after receiving a red card against Gateshead.
[8] In their first season back in the National League divisions, Armstrong led Spartans to a tenth-placed finish, four points adrift from play-off qualification.
[9] In their following league campaign, Armstrong's side saw a huge boost of form, eventually concluding the season in sixth place, qualifying for the divisional play-off quarter-finals,[9] his side failed to continue to the semi-finals after defeat to Altrincham on penalties,[10] having recorded a 2–2 draw prior though extra time could not see Spartans push for victory.
[10] On 21 May 2019, Armstrong was appointed as the new manager of fellow National League North club Darlington,[11] replacing Tommy Wright.
[14] On 6 September 2023, with Darlington languishing at the bottom of the National League North Table, Alun Armstrong and assistant Darren Holloway were sacked by the club.