The 'Britain's Got More Talent' host was asked by the couple to officiate the ceremony in front of friends and family at Rushton Hall in Northamptonshire, England, the location of their wedding in 2008.
Emma, 42, and Matt, 35, wanted pal Stephen, 41, to play a big part in their 10-year anniversary celebrations to make the day super-fun.
In an interview on 'Good Morning Britain', she said: "We'd been planning it for about a year and we were a bit, like, we want to make the most of the whole day but we can't just party all day so let's renew our vows but we didn't want it to be serious, we wanted it to be fun so we asked Stephen Mulhern to do it and he said yes."
Emma - who wore her original wedding dress, designed by Phillipa Lepley, at the ceremony - spilled that Stephen jokingly warned Busted bassist Matt that he would be making a few gags at his pal's expense in his official position.
The mother-of-three shared: "It wasn't like a vowel renewal, it was like a best man speech. He ripped us to pieces, but it was in a great way. He said to Matt beforehand, 'I just want you to know now anything I say is with the greatest love and in the best way.' "
Speaking about why she and Matt wanted to renew their vows as part of their 10th wedding anniversary celebrations, the 'Celebrity Big Brother' presenter said: "On the day when we actually got married, the person that was filming it said, 'Where do you want to be in 10 years' time?' And we said, 'Well hopefully still together, and if we are we'll come back here and have a party, so we did. We went back to where we got married."