Mullin grew up in Oldham, and left school at the age of nine to work in a local cotton mill.
[1] Most cardroom workers in the town were not unionised and were locked out and lost their wages following the Oldham weavers' strike of 1885.
Many of these workers formed a new union, the Amalgamated Association of Card and Blowing Room Operatives,[2] and Mullin was elected as its first general secretary.
[1] However, the Cardroom Amalgamation left the association in 1913 after its member William Henry Carr was not re-adopted as Parliamentary candidates.
Although his health improved enough for him to resume some of his trade union duties, this was temporary, and he died in June.