Sean Lamont made his international debut for Scotland against Samoa in Wellington in 2004. A powerful and committed runner, Lamont has been a consistent threat in a Scotland jersey.

After starting both of Scotland's Tests against Australia in 2005 Lamont was included in their squad for the 2005 Six Nations, winning the Man of the Match award after a strong showing against Italy before crossing for a try against England at Twickenham.

In 2006 Lamont had arguably his finest moment in a Scotland shirt, scoring two tries as France were sent packing from Murrayfield. He was named as Scotland's player of the season soon after. He played in four of Scotland's matches at the 2007 World Cup before a knee injury sustained while playing for Northampton ruled him out of the 2008 Six Nations. He returned to action for Scotland against New Zealand in November 2008 before being included in Frank Hadden's squad for the 2009 Six Nations.

He played only one game during the tournament, against Wales in the opener at Murrayfield, after losing out to Thom Evans and Simon Danielli. A move to the Scarlets followed and in November 2009, under the guidance of new Scotland coach Andy Robinson, Lamont returned to the international stage. He played three November Tests, including a dramatic win over Australia at Murrayfield.

Lamont was an ever-present in Scotland's 2010 Six Nations campaign and played an central role in their successful summer tour to Argentina. Despite only starting three of Scotland's five Six Nations matches in 2011, Lamont was one of Scotland's best performers in the tournament and was selected in Andy Robinson's squad for the World Cup. Following the tournament he announced that he would be returning to Scotland with Glasgow in 2012, following in the footsteps of his brother, Rory.