In March 2018, he was named the vice-captain of Galle's squad for the 2017–18 Super Four Provincial Tournament. The following month, he was also named as the vice-captain of Galle's squad for the 2018 Super Provincial One Day Tournament.

On 3 June 2018, he was selected to play for the Montreal Tigers in the players' draft for the inaugural edition of the Global T20 Canada tournament.

In August 2018, he was named in Kandy's squad the 2018 SLC T20 League. He was the leading run-scorer for Kandy in the tournament, with 312 runs in six matches, including a century in Kandy's final match.

He was named in Sri Lanka's Twenty20 International (T20I) squad for their series against Pakistan in July 2015. He made his T20I debut on 1 August 2015 as the 58th T20I cap for Sri Lanka.

In May 2016 Shanaka scored a century against Leicestershire County Cricket Club in a first-class match on the tour of England. He made his Test debut for Sri Lanka on 19 May 2016 against England on the same tour.^[14] He was the 134th Test player for Sri Lanka.^[15] He took his first Test wicket by dismissing England captain Alastair Cook. Though he took 3 for 46 runs, he only scored 4 runs in the both innings, and Sri Lanka lost the match by an innings and 88 runs.^[14]

He made his One Day International (ODI) debut against Ireland on 16 June 2016. Shanaka scored 42 runs from 19 balls. Bowling, he took five wickets for 42 runs.^[16] He became the twelfth player, and third Sri Lankan, to take five wickets on debut in an ODI.^[17] After the two matches against Ireland, he was named as the player of the series.^[18]

He was re-called for the one-off T20I against India in September 2017.^[19] Shanaka was out LBW for naught in the match and did not bowl. India won the match by 8 wickets.

During the second Test against India, Shanaka was found guilty for changing the condition. The incident was caught on cameras and informed to the match referee David Boon by on field umpires as well as third umpire. By found guilty, Shanaka was fined with 75% of his match fee and handed three demerit points.^[20]

In May 2018, he was one of 33 cricketers to be awarded a national contract by Sri Lanka Cricket ahead of the 2018–19 season.^[21][22]

Shanaka was included to the ODI squad for South Africa series in 2018. However, he did not included to the playing Xi in the first three matches. In the fourth ODI against South Africa on 8 August 2018 at Pallekele Cricket Stadium, Shanka top-scored with 65 off 34 balls, which marked his maiden ODI fifty. The innings witnessed 4 fours and 5 huge sixes with a strike rate of 191.^[23] Shanka along with Thisara Perera breakneck seventh-wicket stand of 109 runs to lift Sri Lanka to pass 300+ in a rain hit day. During the fielding, Shanaka took the wicket of Reeza Hendricks and involved the run out of JP Duminy.^[24] Sri Lanka finally won the match by 3 runs in D/L method, which was their first wi