[2] He joined Scottish Premier League side Hearts as a teenager and eventually established himself in the first-team where he displayed his versatility and even went on to captain the Edinburgh side on numerous occasions before moving to the English Premier League in January 2012 when he signed for Wolves, with whom he made only seven appearances during an eighteen-month stay.

He made his international debut at the age of 19 and is considered to be part of Iceland's golden generation along with the likes of Gylfi Sigurðsson and Kolbeinn Sigþórsson.

He made his debut for Hearts on 20 September 2006, appearing as a substitute in a 4–0 League Cup victory over Alloa Athletic.

He made his league debut as a substitute in Hearts' 1–0 win over Inverness Caledonian Thistle at Tynecastle on 10 February 2007.

On 21 December 2011, Eggert agreed to join English Premier League side Wolverhampton Wanderers on a three-and-a-half-year contract for an undisclosed fee, believed to be £250,000 rising to £600,000.

[15] The player's first team prospects at Molineux failed to improve during Stale Solbakken's tenure as manager, with Jónsson only featuring in one league game as a substitute.

To gain playing time he joined fellow Championship club Charlton Athletic on a one-month loan deal in November 2012.

[17] He did not make any further appearances during the remainder of the 2012–13 season and on 12 July 2013, Wolves announced that an agreement had been reached to terminate his contract with two years remaining.

Late in the summer of 2014 Eggert mutually terminated his contract with the Portuguese side, returning to Iceland to recover from injury.