Sammon started his professional career in the League of Ireland Premier Division with University College Dublin and Derry City.

dubbed him Sammon of college, before a number of impressive performances saw him snapped up by fellow League of Ireland side Derry City in December 2007.

[7] After a short trial, Sammon signed a three-year deal with Kilmarnock in July 2008 for an undisclosed fee,[8] and made his debut on 9 August 2008 in a 1–0 win against Hibernian.

[14] On 31 January 2011, Sammon signed a three-and-a-half-year contract with Premier League club Wigan Athletic for an undisclosed fee,[13] believed to be worth an initial £600,000,[15] after he turned down a move to Derby County.

[20] On 26 December 2011, in a game at Manchester United Sammon was sent off for catching the face of Michael Carrick with his elbow, a decision which was judged as "harsh" by the BBC.

"[26] Sammon made his Derby debut in a 2–0 defeat to Bolton Wanderers on 21 August[27] and scored his first goal for the club in a 5–1 win against Watford on 1 September.

Clough particularly praised his "absolutely superb" performance in the 2–1 win against Ipswich Town on 23 October, where he provided assists for Theo Robinson and Tyson.

[30] Sammon ended his drought with a brace in a 3–2 win against Birmingham City on 24 November and put his improvement of form down to his regaining confidence.

[31] Early in the new year, he went on a ten-game goal drought and fans became frustrated with his performances after missing an 87th-minute penalty in Derby's 1–0 home loss to Crystal Palace on 1 March.

[35] Ahead of the 2013–14 season, Derby signed Chris Martin on a permanent deal after a loan, as well as adding Johnny Russell to the squad.

[36] These additions saw Sammon slip down the pecking order and he was restricted to substitute appearances in the opening stages of the season, though he did start three League Cup ties, scoring twice in a 5–0 win over Brentford on 27 August.

[37] Following the sacking of Clough, Sammon was awarded his first league start of the season against Ipswich, though he was withdrawn at half-time with Derby 4–1 down in a match they eventually drew 4–4.

[38] New manager Steve McClaren quickly signed Tottenham Hotspur's Simon Dawkins on loan, meaning Sammon returned to substitute appearances.

On 27 January, he scored his first goal in a 4–2 home win over struggling Bolton Wanderers, from close range after Andy Lonergan saved from Matt Derbyshire.

On 15 June 2016, Sammon returned to Scotland for the first time in five and a half years, joining Heart of Midlothian on a three-year contract after negotiating an early exit from Derby.

[48] A week later, he scored his first goal for the team, heading Arnaud Djoum's cross as they were eliminated in a 2–1 second-leg loss at Tynecastle.

[55] He made his debut for the Jags later that day, playing the full 90 minutes of a 1–0 home loss to reigning champions Celtic.

[61] He made his debut nine days later in the Scottish League Cup, scoring a last-minute equaliser in a 1–1 home draw with Livingston, though his team lost on penalties.

[69] On 22 March 2013, he made his competitive debut in a 0–0 draw away to Sweden in a World Cup qualifier, coming on as a substitute after 87 minutes.