Assalamu alakyum, 

I witnessed a marriage ceremony some time ago and I wonder if it is valid or not. I do not have much knowledge on this issue so I would like to ask you. 

The marriage proceeded in the following manner: There was the bride, which is a convert Muslima, and the groom, the imam of the local mosque and two male Muslim witnesses. The imam told something about matrimony and then began with the ceremony. He said something like "would you marry me...following the Sunnah of the Prophet...with the mahr of..."  I do not remember his words exactly... and the bride had to repeat this words, then he spoke something like an acceptance, and the groom had to repeat the acceptance. Then all signed the marriage contract. 

Is this correct?

In the name of Allah, the Most Merciful, the Grantor of Mercy

All praise belongs to Allah, and may prayers and peace be upon Allah’s final Messenger.

The marriage is valid according to Imam Abu Hanifah. According to the rest [the other major madhahib], a woman may not conduct the marriage contract herself, but her guardian does that. 

Since she is a new Muslim and may not have a family member that is Muslim, the imam of the masjid could marry her off, but he should have been the one to conduct the contract with the groom as the other party. The first contract is legally consequential in the sense that their relationship was not illegitimate; however, they should repeat the contract with the imam being the other party acting on her behalf.

Allah knows best. 

Related fatawa:  85654, 85796, 86929, 86983, 87189