In February 1905, after 167 league games, he left West Brom to join Aston Villa for a fee of £250, but played just 11 times before joining Nottingham Forest in April 1906. A year later he moved again, this time to Southern LeagueSouthampton. According to Holley & Chalk's "The Alphabet of the Saints" he was "a methodical yet energetic half-back, dedicated to looking after the forwards by supporting, feeding and directing them into advantageous positions".
Hadley's first managerial job was with Merthyr Town where he was appointed in May 1919. He oversaw their promotion to the Football League in 1920 and guided them to 8th place in Division Three (South) in their first season. He left Merthyr in April 1922 to manage Chesterfield, but left in August the same year.
In November 1927 he took over at Aberdare Athletic, who had failed to gain re-election to the Football League the previous season. He left in April 1928, returning to Merthyr Town as manager, but left again in November the same year. He began a third spell as Merthyr manager in 1930, leaving in September 1931.
His final managerial job was with Bangor City who he joined in July 1935 and left in April 1936 when he retired.
Hadley's brother Ben also played professionally for West Bromwich Albion, but had left before Harry's arrival at the Hawthorns.