After passing the grade 5 scholarship exam Lahiru joined Prince of Wales' College, Moratuwa and played in the Under 13, 15, 17 and 19 team.

[12] He made his Test debut against England at the Rose Bowl in June 2011,[13] coming into the side for the injured Tillakaratne Dilshan.

Thirimanne reached a deserved century by cutting Xavier Doherty backward of point for the winning runs.

Also the 137-run stand between Thirimanne and Tillakaratne Dilshan is the second-highest second-wicket partnership for Sri Lanka against Australia.

The 203-run stand between Chandimal and Thirimanne is the third-highest fifth-wicket partnership for Sri Lanka in Tests against Bangladesh.

[17] During the 2014 Asia Cup in Bangladesh, he opened the batting with Kusal Perera, due to the injury of Tillakaratne Dilshan, who was not included in the series.

He scored two magnificent hundreds against Pakistan, where he finally ended up being the highest run scorer in the tournament with 2 centuries with the average of 55.80.

During the 2015 ICC Cricket World Cup, Thirimanne started to open the batting with Tillakaratne Dilshan.

Before the tournament, the Sri Lankan opening pair was struggling to pick up, but finally, it was sealed between Thirimanne and Dilshan.

With many injury concerns within the Sri Lanka squad, Thirimanne was recalled for the second Test at SSC, although he did not feature in the first XI.

[29] This stand ranks as only the second instance where fourth innings of a Test match in Asia scored more than 150 run opening partnership.

[32] His test hundred was also his second in Galle, having hit an unbeaten 155 against Bangladesh in 2013 - Thirimanne was going for a Sri Lanka-record 54 innings between tons.

His boundaries, rare as they were, were nevertheless exquisite in their execution - be it a back foot punch through cover point, or textbook drives through mid-off.

He accomplished this by simply refusing to attack deliveries outside off, thus forcing the bowlers to adopt straighter lines.

It was his third consecutive half-century and in the second innings he put on century opening partnership with Dimuth Karunarathne and Sri Lanka managed to draw the match.

[40] During second test match, Thirimanne and Dimuth Karunaratne added 209, Sri Lanka's first 200-plus opening stand since 2011, and the first in the country in 21 years.

The partnership, which is also the pair's third consecutive century stand, ended when Karunaratne was caught behind off Bangladesh's debutant Shoriful Islam shortly after tea.

[42] Thirimanne captained the cricket team for 2014 Asian Games at Incheon, where Sri Lanka won the gold medal by defeating Afghanistan in the final.

In 2019, ten Sri Lanka players including permanent ODI and T20I captain withdrew from the tour of Pakistan citing security concerns.