Q. If the Mohel left the minyan (because he had to go to a Bris), before the beginning of the repetition of the Shemone Esre, do you say Tachanun?
A. Taz (O.C. 131,10) quoted in Mishna Berura (ibid. 26) recommends that a groom should not come to Shul during the seven days after his wedding, so not to exempt the attendants from reciting Tachanun. Ridbaz (responsa 1,179) and Trumas Hadeshen 2,80) construe from this that if the Choson was in Shul during Shemone Esre but left before the beginning of Tachanun, it is recited. Toras Chaim (ibid. 7) and Eishel Avrohom advise that it is better that the Choson should attend the Minyan and exit before Tachanun. However, not all Poiskim agree, (Hilchos Ktanos 1,21- Tefiloh Kechichoso 15,12 – Rib'bos Efroim 5,8) maintain that if the groom was present at the beginning of the Amida and then left, no Tachanun is said. All this would equally apply to the presence of a Mohel.
Horav Shlomo Miller's Shlit"a opinion is that it is better to recite Tachanun.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit"a