English League Two club Tranmere acquired the services of the 23-year-old goalkeeper from Scottish Premiership side Celtic on a season-long loan in the summer.
Putting pen to paper on a season-long loan deal with the Merseyside-based club saw Doohan reunited with Mellon, who he worked with at Dundee United towards the end of last season.
Doohan, who made two appearances for Dundee United last season, has revealed that he got to know Tranmere boss Mellon during his time on loan with the Scottish outfit.
The Celtic loan star disclosed that he was informed Mellon wanted to take him to Tranmere having been impressed with him at Dundee United, leaving him delighted.
“Dundee United, I was there for the last five or six weeks of the season on an emergency loan, so I got to know him there“, Doohan told Tranmere’s in-house media.
“Obviously, got the call in the summer [saying] he wants to bring me down here.
“It made me happy that I impressed him at Dundee United and wants to bring me to his next club.“
Doohan replaced Joe Murphy as Tranmere’s starting goalkeeper after the 40-year-old suffered an injury this month and will be looking to keep his spot in the team.