Mullin broke the League Two goalscoring record last season with 32 goals in 46 games for Cambridge and helped them win promotion to League One.
Mullin had many suitors from the Football League but the ambition of the National League club laid out by Hollywood star McElhenney convinced the 26-year-old League Two player of the year to drop down two divisions.
Mullin told the club’s official website: “The ambition of the club attracted me here.
“Rob McElhenney gave me a call a couple of nights ago – at that time, I wasn’t too sure about making the move – but once he outlined the plans for the club, and the plans for me in the future, it was something I really wanted to be a part of and something that just excited me that much that I had to come.
“He really sold the club to me. What they have planned for the club is magnificent and something I want to be a part of.”
McElhenney’s fellow co-owner Ryan Reynolds also welcomed the striker to North Wales.
Reynolds posted on Twitter: “Hello, Paul Mullin,” accompanied by a Welsh flag and a delighted Mullin responded: “Hi Ryan Thanks for having me pleasure to be here.”
Mullin previously had stints at Morecambe, Swindon and Tranmere before signing with Cambridge permanently last year, where he also broke their single-season goalscoring record in all competitions with 34 goals in 50 appearances.
Wrexham manager Phil Parkinson added: “I’m delighted to welcome Paul to the club. He comes in after a terrific promotion season with Cambridge last season.
“He obviously brings a goal threat but most importantly he’s a team player. He brings a great work ethic and the mentality which we need at the club.
“He brings the goals, but equally Paul’s attitude and work ethic is important for what we are trying to build here at Wrexham AFC.”