The 29-year-old, who made his Wrexham debut in 2021, appeared as Welshpool, a Welsh version of Deadpool, in the latest Marvel movie Deadpool & Wolverine.
Mullin has made 122 appearances for the Red Dragons and has scored 88 goals in the last three years.
In a post on X, Reynolds said: “You may have noticed one member of the Deadpool Corps was far deadlier than the rest. WELSHPOOL.”
But Mullin wasn’t the only Wrexham AFC player to make an appearance as teammate Ollie Palmer also cameoed as a bar goer.
The latest Marvel movie netted $205m (£159.5m) at the box office in its debut weekend in North America – securing a spot in the top 10 openings of all time.
North Wales-based club Wrexham, which was bought by Reynolds and Rob McElhenney in 2020, was promoted for the second season in a row earlier this year.
Wrexham will play their first League One game since achieving promotion on 10 August.
The trials and tribulations of the club have been documented in Disney+ series Welcome To Wrexham.
Country singer Dolly Parton was recruited to help promote the latest series after she found out about her Welsh heritage.
Transport for Wales announced in April that it was naming two of its trains in honour of Reynolds and McElhenney.