He made his international debut on 11 June 1995 in a 1–0 defeat to Ukraine in Kyiv during the UEFA Euro 1996 qualifying, coming on as a substitute for Dubravko Pavličić in the 27th minute, following a red card to Tonči Gabrić.

He made his second appearance in the qualifying on 3 September 1995 in a 7–1 win at home to Estonia, coming on as a substitute for Dražen Ladić in the 29th minute.

Prior to the UEFA Euro 1996 finals, Mrmić appeared in three friendly matches, also playing the full 90 minutes in a goalless draw against England at Wembley.

Mrmić went on to make three appearances in the 1998 FIFA World Cup qualifying, which included playing the full 90 minutes in the second leg of the playoffs against Ukraine.

He was also part of the 22-man squad that won the bronze medals at the 1998 FIFA World Cup finals, but did not make an appearance during the tournament.