The defender penned a three-year deal yesterday after the U’s agreed an undisclosed fee with Rotherham United.
It marks a return to the club for Mullins, who made a big impact during an eight-game loan spell last season.
United came close to making the move permanent, but an injury crisis at Rotherham prompted a recall and Mullins went on to captain the Millers to automatic promotion.
But patience paid off for Wilder, who became aware the 27-year-old was available earlier this summer, and the U’s boss believes the signing has completed his summer business.
He said: “I’m delighted. We have worked extremely hard to attract the right calibre of players and Johnny Mullins really caps off a good summer of recruitment for us.
“He’s popular with his fellow players and is well respected by the staff when we worked with him first time around.
“We were not a million miles away from securing him permanently a while back and I’m sure it will be well received by the supporters.
“We always kept an eye on certain situations and when we found out he might be available we worked hard.
“He’s come down a league, but I think it’s a measure of how we looked after him and how he enjoyed his football here that he wanted to come back and join us.”
Mullins, who did not celebrate scoring against United in Rotherham’s 4-0 win at the Kassam in March, had no problem with dropping down a division.
He said: “I had a great time when I was here last year, so I’m looking forward to pulling on the yellow shirt again.
“Having fought so hard to get Rotherham up last season, I wasn’t planning on playing in League Two again.
“However, I believe that this time next year Oxford will be a League One side anyway and I’m looking forward hugely to helping make that happen and then moving onwards and upwards with the club from there.”
The former Reading apprentice will be assessed at the training ground today, but may not feature in tonight’s friendly at Oxford City.