And now it has all become clear - he did it to star in an advert with Hollywood superstar Ryan Reynolds.
Reynolds, who is part-owner of Wrexham with fellow actor Rob McElhenney, has posted numerous funny videos around the Wales-based club - but none quite like this.
The advert, which is for security company 1Password, starts with Reynolds at his side's stadium, The Racecourse Ground.
He then goes into the changing room to address the Wrexham players, who are currently fourth in the National League table and pushing for promotion.
To his surprise, all of the players had already downloaded the 1Password app, however, when Mullin - who has 16 goals this season - calls him an 'idiot' under his breath, Reynolds loses his cool.
"Mullin," Reynolds screams as he goes head-to-head with his star striker.
"I will shove a red card so far up your a***, people will think it's your tongue", the star claims.
Mullin, who, for now, is the third top scorer in the English fifth-tier, may be in contention for a role in Reynolds' next film with his superb amateur dramatic skills.
Who knows how many takes it took, with Mullin somehow managing to keep a straight face as the American owner confronts him in the hilarious advert.
And if the goals dry up, don't be surprised to see the 27-year-old star in the next Deadpool movie alongside Reynolds. Stranger things have happened…